So I picked up a Model M on Ebay. Didn't ask the seller to check the part number because it was a square badge, Queen's English board with "tested and working" in writing. Good enough for me. It arrived a few days ago and the markings on the back are quite odd.
First there's a sticker that says "IBM Internal Use Only". Then there's two stickers, both dated to 3/12/85, with two different part numbers. The first is 1386714 (I think, there's a scratch over the second digit). There's no record of this number on Wikipedia's list of Model Ms or the Deskthority list. The second one says 1390136. This ones on the Wikipedia list (although they haven't found a 1985 one before) and the Deskthority list.
Has anyone encountered a Model M with markings like these before? I'm a bit confused.