Square plug behind oil filter adapter?????

My machinist took the plug out that was in the side of the block where the 90* oil filter adapter goes on the block. The plug has a square in it for a special tool to remove it. It isn't 1/4" or 3'8". I need to measure it, but I am guessing it is a 5/16" square. Where would I get something to fit that? Never seen anything that size.