I have several Model M13s and can confirm this converter works fine with them:
https://www.amazon.com/Perixx-PERIPRO-401-PS2-Adapter-Built/dp/B008DFVQFW/. I've been using two of them for almost two years without any issues.
Got my response from Unicomp. Pretty underwhelming, so I guess an adapter is the only option. As I said before, best combo for a Model M13?
I'm not surprised. With the exception of the Mini M, all Unicomp keyboards use a different style of connection between the membrane and controller since Unicomp keyboards are largely based on the P/N 42H1292 (gen 4 IBM/Lexmark era) Model M design. Vintage (gen 0 to 3) Model Ms use flex connectors called triomates, whereas most Unicomp keyboards use a pressure connection where the controller PCB is held against the membrane's connector.
For context if you're curious, old style (
source):
New style:
FYI: the old-style controller pictured above is not an M13 PCB, but the overall principle of how the controller interfaces with the keyboard stands (the M13 would look similar but with extra headers/connectors for the mouse passthrough port and the TrackPoint II module). Which is why the controller Unicomp makes for their EnduraPro will not work with your pre-Unicomp M13, unfortunately.