rubber plug in rear diferential?

Just trying to wrap up my reconstruction of a 93 cherokee and got around to checking the fluid levels in the differentials. Front one had a 3/8" drive plug in it. the rear had a rubber plug going into a hole with no threads at all. I've never seen nor heard of anything like it. It was full, but it doesn't appear to seal very well either. Is this a common setup? I may be able to swap out a cover from a different Cherokee if it's not the same thing. Is there a purpose for this?

normal set up, fluid should be 1/2 under hole so should'nt leak