Help Decoding this 8 3/4

64 or earlier, likely a b-body. The nut holds the axle and drum together,(that is what is known as a tapered axle, it doesnt have a normal flange) and can be a real ***** to take apart, especially without the proper puller.
There is lots of info on "for b-bodies only" for disassembly and conversion. Do a search. General driveline, and also 62-65 threads.