I understand that the M1's fuel in the rear but what are the two caps for on either side of the drivers hatch? I cruised the web and haven't found any answer........

An M1 series tank has two rear and two forward fuel cells...those are the caps for the forward cells

I thought that might be the case, although there is only one cap on the rear. I appreciate the info.......

You must be building an M1A2 SEP then. On the M1A2 SEP, the left rear fuel tank was removed and replaced by either an aux generator, or the Hawker Battery System (a bank of aux batteries) to run electronics and such so the crew doesn't have to run the main turbine, thus saving on fuel.

You are good !!!!

I assume then that the TUSK and TUSK II are using the same system?

No, you are confusing a bolt on armor system with a specific version of the Abrams tank.

TUSK I or TUSK II can be fitted to any Abrams tank.

The APU/Harker Battery System is only part of the M1A2 SEP and M1A2 SEP v2.

Damn, things were soooo much simpler back in the olden days when I crew'd on a CEV !! I think the only crazy change was the M-60A2, the Starship" I think it was .........