[Lynn Eades]

Hi, What ever it is, it is serious. I don't know about the M37 specifically, but there is a big bolt in the side of the earlier diffs. That bolt supports a bronze thrust pad against the back of the ring gear (crown wheel)This is what it looks like to me. To be in this state something is not good.
If it is not that, then it will be a a thrust washer from the side or spider gears. These are usually steel, with a bronze wear face on them, and so is unlikely. There shouldn't be any other bronze type material anywhere else in the whole thing. Having the bronze in there is the reason why you cannot use E.P. oils in the diff. (it attacks the bronze)