Completing a single player war story with all letters found and challenges completed will unlock a unique weapon/skin for use in multiplayer.

Unlockable	How to unlock
Coupe Coupe (Melee Weapon)	Complete Tirailleur with all letters and challenges.
Cricket Bat (Melee Weapon)	Complete Under No Name with all letters and challenges.
Soveig's Knife (Melee Weapon)	Complete Nordlys with all letters and challenges.
The Last Tiger (Tank Skin)	Complete The Last Tiger with all letters and challenges.