Taylorswiftttttt kindly proxied one of these keyboards for me! I cleaned it up, bolt-modded it, and changed the modifiers to something more ANSI-ish in the process. Pardon the potato pics!
In my research about the keyboard, orihalcon
suggested an 8-pin SDL IBM 54P8828 cable might work one of these, which I purchased. If it works, I'll post a pinout and convert the keyboard with a Teensy.
The membrane has contacts for an ANSI pipe key as well as Alt keys, but the blockers on the case are too wide for the latter keys to fit, sadly. It was definitely made in 1988 as well, the date was deceptively misprinted. Compare the 8 to the 9 character in the plate part number.
The keycaps were by far the most interesting part of this keyboard. The popular belief is that Unicomp uses the same old molds as IBM/Lexmark did. I did not believe this was true. Their caps had a sprue mark location was different (to the sides rather than centered), mold markings/recesses at the intersections of the interior walls, mold numbers on the ceilings of some keycaps, and softer/more rounded edges (this being due to "deteriorating" molds). Since no other IBM keycap I had seen up to that point had these characteristics, I always assumed Unicomp did indeed retool at some point. But I was wrong.
The keycaps (the pearl ones at least) of the 4683 keyboard use what appear to be the same molds that Unicomp uses today! The 4683 and Unicomp caps look nearly identical, right down to the rounded edges.
Below are some comparison photos of a 4683 keycap with one from an IBM Model M 1390120 and Unicomp Ultra Classic.
4683 (L) and 1390120 (R)
Unicomp (L) and 4683 (R)
EDIT: As an addendum, I was just reading the wiki's entry on the
Model M when I saw Daniel Beardsmore's photos of his 1996 Greenock Model M. The keycaps on those seem to use the same mold as well. I've never owned a Greenock M so I did not know about these either. Good to know, still.