The manual for the M-80 shows how to mount the rack ears. I have my M-80 and a couple of C-80s mounted in a Yamaha rack. Actually, I won the ears on eBay and the seller asked if I was interested in the rack. Of course I was.
I don't think many of these amps were sold with the optional ears. The ears are either quite rare or at least are not sold often.
Happy listening to all