The Army Internal Correspondence is a historical item which can be found by Karl in Reims Forest during Chapter 3.
Location[]
After distracting the soldiers with apples and turning the wheel to enter the building, find this item in the locker to the right.
Description[]
"A special card that soldiers were allowed to send to each other. Brothers could thus correspond with each other even if they weren't stationed together. If one or several family members died, demobilisation was not an option. In some cases, a whole family of men might be wiped out."