After a long search, I finally found an Intel 8231A math coprocessor not offered for hundreds of dollars by scalpers. He had 4 so I bought them all. And surprisingly, they all work. I built a coprocessor board in the simple configuration (async) and just as surprisingly, it also worked - not something guaranteed with some of my projects.
But... While all four ICs work the same, they have some subtle differences with the official Intel spec sheet. The stack on the chip pushes and pops in a strange manner or not at all and contains other data that is easily identified, but shouldn't be there. Assuming that the defugality was caused by my design or circuit, I spent quite a while with a logic analyzer watching data come and go, finally deciding that the interface was fine.
The specs sheet is labeled 8251A just as the chips themselves, so the difference is strange. Unfortunately, despite long sessions with DuckDuckgo and even Google, I can find nothing anywhere besides that single spec sheet. I have all the original Intel manuals from that era and the 8231 is not referenced anywhere. That may be because it is not really an Intel IC, but was purchased from AMD and rebranded.
By lots of testing, I know how it works even if it doesn't match the specs and can now write a library for it. But, anybody ever used one? Or better yet, know of some website that might have an ancient article along the lines of "Programming the Intel Math Coprocessor, or "The weirdness of the 8231 explained."
???