Like Louie said, it is where the slave receptacle goes, for providing external power for jump starts, etc. The little slot next to it is where the master electrical power switch should be.
Attached pictures should give a good idea of what should be there.
Just FYI, the engine will begin to overheat fairly rapidly if run with the panels off for long.
One thing that helps keep the operating temperature down, particularly when idling, is opening the front radiator panel (red arrow in pic 2 attached). I learned that trick from Louie (Baseballump) and the other guys who drove these in Vietnam.
If you have the hull side doors closed, it also sucks air past the driver through his roof hatch, at a surprisingly high rate, which aids in cooling him (her?). For us in the southern states, this can be important!
Cheers
PS - the reduction in size of pic 1 blurred it out some, if its not clear enough I will go take a better one to illustrate.