It's a 75/46-mm AA gun (Model 34) mounted on a truck
http://www.lonesentry.com/manuals/ar...ppendix-d.html
75/46-mm AA gun (Model 34) Caliber (inches)2.97 Muzzle velocity (foot-seconds) 2,350 Max range (yards) 14,100 /27,200 ft Remarks "Mobile gun "
Somebody long ago decided either this or other information warranted it's inclusion. It's sat in dusty obscurity until you bought up the issue of AP ammo. It will have AP ammo in the next patch release
Don