If you collect every historical item you find along the way, then you'll unlock this achievement in Chapter 2 in the 'Ypres' act.
Chapter 1: Dark Clouds
1.) German Helmet: Found after 'defeating' the first training dummy during basic training.
2.) Tinder Lighter: Before raising the flag at the end of basic training, look on the ground to the right of the flag pole.
3.) Stationmaster's Whistle: During the train section where you need to fend off some soldiers pushing around another soldier, run to the right to grab a wine bottle, then go back left and chuck it at the bell above the conductor
4.) Letter from a French Soldier: As you progress to the right of the same area, you'll find a soldier standing to the left of the French flag on the bottom. Examine this soldier.
5.) Metal nib: When you climb on top of the train, go to the right first to find it on the edge of the train car.
1.) French Identification Tag: Go a bit to the right and you'll see an overturned wooden cart. Examine behind this cart to find it.
2.) Gold button from a French Uniform: You'll come to an area where you need to toss a grenade to destroy some rubble blocking a cart. Examine around the rubble before tossing the grenade.
3.) HQ Map: You'll come to an area where you'll climb down a ladder on a bridge and see some dynamite placed. Go to the right after climbing down this ladder to find it at the edge of the area.
4.) German Cartridges: After blowing up the bridge and before lowering the rope ladder, examine around to the left of the water to find this item.
5.) Embroidery: You'll come across a building where you need to knock some soldiers out. There's a rope ladder near the first one. Defeat him and climb the rope ladder and break the wall to the right to find it.
6.) Letter from a German Soldier: Once you reach the roof on this building, you'll have to knock out two soldiers. Knock out the one on the right first and then go to the edge of the roof on the right to find this.
1.) Photo frame: Once the level starts, go all the way to the left to find it on the ground.
2.) Valiant Stories Contest Memorabilia: You'll eventually start digging into trenches. When you dig into your first one, climb the ladder down to the bottom and dig to the left to find this.
3.) Crucifix: As you make your way through the trenches, you'll see the Crucifix on the right side of some dirt that has some shells inside of it. Exit the trench like normal and proceed to the right until your dog starts digging at the ground. Have him dig up a bone, take it, and go back left and chuck it at the rope ladder. Climb up and dig out the Crucifix.
4.) Compass: Upon reaching the next trench, you'll come across a circular area you can dig in that has shells in them. Carefully dig up and to the right to get to the Compass safely.
5.) French Coins: As you proceed you'll eventually start playing as Freddie. Proceed until you reach the entrance to the next trench. Right outside the entrance are the French Coins.
6.) Khukuri Knife: You'll have to solve a dynamite puzzle in a trench by tossing certain dynamite to the soldier on the scaffolding. Once you set all the dynamite and blow up the ceiling, go up the ramp it forms and you'll see the Khukuri laying on the ground in the building you emerge in.
1.) Canadian Identification Tag: You'll come across an area in which you need to dig downwards and you'll come across a box type puzzle. Get the box to your level and climb across to the left side to get this item.
2.) Urine Soaked Cloth: After getting to the bottom floor on the box puzzle, you'll be greeted with another puzzle that involves turning valves to make the gas flow stop. On the far right of this area is this item.
3.) Letter from a Canadian Soldier: After you turn off the gas and climb back up to the surface, instead of going to the right with everyone else, go left first and up a hill to find this.
4.) Gas Canister Deployment Strategy: As you proceed, bombs will start dropping on you and you'll run into another trench. The item is at the entrance of this trench.
5.) Belgian Shako: When you come across a puzzle that involves a basket, keep going right and your dog will start digging at the ground. Command him to dig to uncover this item.
6.) Pocket Watch: Once you drop Freddie underground with you, run to the right and cut the barb wire. Gas will start leaking behind you so quickly grab this item right after cutting the barb wire before it catches up to you.
1.) Periscope: You'll come across an area in which you have to hide behind wooden boards as German soldiers run left and right. As they run to the right, go left and you'll reach a dead end. Have your dog go under the barricade here and fetch the item on the other side.
2.) Infantryman's Mask: You'll reach a part in which you need to have your dog get a pair of wire cutters for you. Once you get them and cut down the barbed wire in your way, climb down the ladder and dig to your left to find this hidden in a corner.
3.) Vial of Nevrostenine: You'll need to have your dog distract a soldier so you can knock him out. Once you do, enter the trenches and go all the way to the right into the bunker to find some crates stacked on top of each other. Smack them and the item will drop down.
4.) Letter from a German Soldier: You'll need to use a minecart to sneak past a machine gunner in a nearby tower. Upon doing so you'll have to dig into another trench. Enter the door down here and look on your right near some barrels to find this item.
5.) Tin of Sardines: In this same trench there's a guard on the far right guarding a door. Distract him with a wine bottle and knock him out. Enter the door and look on your right to find this.
6.) British Coins: Once your dog is down in the trenches with you, go all the way to the left to find a locked door and you'll see the keys on the other side of the barricade. The British Coins are also over on that side so have your dog fetch them for you.
1.) Safety Razor: Go to the right to find a building and a small puzzle. Upon completing the puzzle, get to the top floor of this building and examine the cupboard.
2.) Overalls: In this same building, enter the doorways so you wind up in the basement and look on the left side to find them.
3.) Advertising Brochure: As you run back into the street and head left, your dog will start digging behind the car in the street. The item is in this dig spot.
4.) War Godmother Certificate: You'll need to use a pair of clippers to bring down a wooden beam to rescue a man in a burning building. When you do, have your dog go across this wooden beam to retrieve this item.
5.) Early Gas Mask: When you're back in the left building after cutting down the wooden beam, go all the way to the right in this building and examine the cupboard here to find this item.
6.) Active Service Pay Book: After saving the men in the city, you'll be able to push that giant stone block that's covering a hole to drop down in. Once you're in this hole, go all the way to the right past the ladder to find this item.
Chapter 2: Broken Earth
1.) Inkwell: You start in Anna's apartment. Open the cupboard to find it.
2.) Broken Watch Strap: You'll have to grab some coal and hit a bird with it for it to drop a key. Before doing so, look at the far end of the screen to find some birds huddled around an object on the building across from you. Chuck a piece of coal over there and it will fall into the street. Pick it up by the sewer grate when you go down into the streets.
3.) Letter from a Belgian Civilian: After getting the key from the bird, unlock the door and look on the dresser to the left of the doorway you came out of.
4.) Newspapers: Run all the way to the right in the street to find the Newspapers hiding under a fold out sign, next to a car that's on a jack.
1.) Shrapnel Shell Remnants: You'll have to heal a man that was stuck under a horse. Upon doing so you'll enter the next area. In this area go all the way to the left to find this.
2.) Nurse's Manual: As you run to the right, you'll find a water tower and the medical tent. To the left of these you'll find an opening in the gates. Go through this opening and go the left to find this. The horse's head may obstruct your view but just examine the area and you'll find it.
3.) Tallow Socks: Go back and enter the medical tent now and examine the locker on left side of the tent.
4.) Mess Tin with Holes: Exit the medical tent and go right to find a medical truck. This item is underneath the front wheel.
5.) Letter from a Prussian Soldier: You'll come across a bridge as you continue to the right. Climb the ladder down and go left to find this.
6.) Lice Comb: Climb back up the bridge and go all the way to the right to find this.
1.) Letter from a Scottish Soldier: As you venture to the right, you'll find a building with red bricks. Enter this building and go left and head outside to find this.
2.) Wooden Top: You'll notice a tree to the right of the item you just picked up. Climb up this tree, break the window and help the woman inside. You'll automatically obtain this.
3.) Broken Walking Stick: Once you knock the cog off the roof of the church with the church bell, go down and follow the path to find a lever to turn off some gas. Climb the ladder here and help the man to automatically obtain this.
4.) Bedpan: As you continue your way through the streets, you'll find another building. Go through the doorways leading upstairs until the walls are red. There's a cupboard in this room containing the item.
5.) Stereoscopic Camera: While you're still up here, go right and you'll find a painting on the wall. Hit it and grab the keys here. Go all the way downstairs and go right, pushing the drawer so you can climb to the ledge and climb outside. Go right and unlock the gate and hit the painting in here to obtain this item.
If you got every item up to this point, then the Stereoscopic Camera will be your 50th item collected, popping the achievement.