This may have been the first device that was created with the intention of it being used as a logic device -- but small PROMs were being used to replace block of combinatorial glue logic on the early 1970s -- and I remember using them as small look-up tables to configure circuit boards and as a way to fix problems without rewiring the boards in the early 1980s, so that makes them PLDs in my book (grin)
Hi Max, you didn't come up with the answer to my question, but you certainly covered the background! When I asked the question, I wasn't thinking about PROMs, but I certainly remember using them in a programmable logic way (a 100+-bit state machine)! Your mention of MMI brought back memories. Your history does show that Altera showed up on the scene later than some other manufacturers though. I'm hoping others can add to the discussion.