As soon as the mission starts but before boarding the tank, turn around from your starting position and go to the left, towards the big tree with several oil barrels. The lighter is found on top of one of the barrels, behind a standing soldier.
Before reaching the AA gun itself, there’s a small storage shed to the left of the field, near a broken tractor. You can find the pocket watch on the table.
After using the AA gun to clear the airspace of German bombers, you’ll have to eliminate the artillery crews inside the trenches. Then, you’ll have to push forward with your tanks. When you reach a dug-out bunker/tunnel, enter through the middle opening to find the ally soldier in trouble. (2nd Heroic Action) From there, continue to the other end of the tunnel to find a dead German soldier. Loot the memento from him.