M46 fill plug
 

The transmission on my '84 242 GLT (M46) works fine but I usually top it off with fresh fluid every 3 months or so because it has a slight leak. The problem is that the fill plug is a bit buggered up. I have been looking for a replacement but can not find one, even from the dealer. It is the square headed plug made out of brass. The dealer keeps getting me one that is too small.

Ideas?