ammo can

[browningautorifle]

said that they were for armoured fighting vehicles

That's the bulk packaging for belts for the GPMG, ours are wood crates, or they used to be anyway. Four belts is normal.

whats this mechanism on it

Locks it onto side of the mount of the .30 cal M1917 Heavy MG. Here's one versions and a link to describe...

Unusual WW2 .30 cal ammo box - ORDNANCE - U.S. Militaria Forum