Normally a studio master is the same as a production master. That is a copy of the original master tape which is sent to the mastering engineer who then uses it to cut the lacquer for a vinyl pressing or possibly a CD or cassette release. Along with the production master is sent a safety master which is identical and serves as a back up to the production master in case there is a problem. It is not a commercial release. Normally they emerge from the collections of the engineers who sometimes keep the masters for their own collection and eventually may sell them to others. If you look at the Thriller listing on Discogs, there were a ton of releases done in different countries and normally those would have each been pressed in the country using a production master made from the original master tape. So there are probably quite a few floating around in different collections.
Provenance is very important and the dealers I buy from normally have trusted sources, often engineers in different countries. They normally get safety masters, rather than production masters, since the record company's policy is typically to have the production master returned to them, but the safety master is often kept by the engineer as a souvenir of the job. You will see tapes on ebay advertised as masters, or safety masters, or production masters. I NEVER buy from ebay. Vast majority of those tapes are fakes - dubs from CDs typically, or even sometime from vinyl (usually after processing digitally to get rid of clicks and pops). There are ways of telling if it is genuine, but the clues are not certain. For example, if you digitize a tape at high rez (say 192/24) and then examine the waveform (I use Izotope RX3Advanced), if the signal cutoff is extremely sharp at 22kHz, then it is almost certainly sourced from a CD.
Finally, commercial tape releases from the old days were done at 7.5ips or sometimes 3.75ips, and almost all were 1/4 track stereo tapes, duplicated at very high speed (16 times normal speed or even higher). The masters were all duplicated in real time, normally at 15ips 1/2 track (sometimes even on 1/2" tape).
Larry