was trying to change diff lube in rear axle but can't remove plug. It's like none I've seen before. There seems to be a raised boss on 1 flat that seems like it.s supposed to be a register for a tool and the plug is hollow with screw threads on the inside. Any help please. I wonder if one of the prior servicings someone striped a plug and put in a substitute hanging around. I've worked on a lot of cars and never seen this before. The drain and front axles don't have this...just the usual style of plug.