I'm still banging on this and getting nowhere. I notice that when I go into a normal M1A1 and man position, I get the ammo counter along with fuel and armor bars. The turret direction is above that, where it colors the tank's frame according to where damage has happened.
Somehow my own single-player adaptation of the M1A1 is missing all this. I have the ammo counter but that's it. Does anybody need screenshots?
I was getting the impression that some of this stuff is assigned in Oxygen, but I didn't find any suspicious fields, other than something about an RPM gauge. Wholesale pasting the gauges from the cfg_vehicles.hpp file didn't seem to work for me -- I may have done something stupid but it just wouldn't work.