I have found over time and trying old and new Ms that the differences are pretty minor, for example the key feel of a 2001 Unicomp "401" I got recently was among the very best of my collection of "M" boards. An IBM/Lexmark built 1994 M13 I just got is awesome - better even than my excellent MaxiSwitch M13 Black. I also have gotten over my bias/snobbery about "only IBM" is the "one true Keyboard". I think that all the production Model M runs vary a bit, and the amount of wear on a used board must be factored in, too. In short, it's not that simple. Just buy a Model M - old IBM, newer Lexmark, or brand new Unicomp - it will still be better than most everything else out there. You can try the variations until you are satisfied that it doesn't matter - like I did. TPF