While a DC socket would not stick out so far, and therefore be a possibility, it seems to me that the bolt pattern for them was more squarish, whereas this one is more the shape of a slave receptacle.
The engineers in Ottawa made a mistake on the M109s and installed a slave cable outlet onto the back of the turret. The idea was that a small 2kw generator, mounted on the back of the turret, would keep the batteries charged up rather than having to run the main engine of the M109. Only problem was that the troops found it much easier to hook their slave cables to that outlet. The turret ring commutator brushes were not made to have 300 or more amps run through them on a cold start. A DC receptacle would have been the correct item to have been installed there, but the bolt patterns were not the same so there was no easy fix.
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