changing rear diff fluid

I think its a 12mm bolt head for the following reasons:

1. It would be very rare to find a bolt head or nut with 13mm
2. if 1/2 fits (12.7mm) a bit loose then it wouldnt fit on a larger 13mm.

I could be wrong but from experience in metric system a 12mm is much more common.

Hmm... The plug has an inset square end. ("inny" or "female")