Gregory's Server@jonty @cstross I'd suggest IBM bus and tag cables, from their older mainframes: https://upload.wikimedia.org/wikipedia/commons/c/ca/IBM_360_Bus_Tag_Cables.png. Note that the connectors are about 10 cm long — and you needed two to connect to each I/O device…
@SteveBellovin @jonty @cstross
Beat this moose to it. (I may have a couple somewhere - unless I left them in my desk when the COVID-19 lockdown started and then turned into my retirement from Dinosaur Herding).
Plessey Mk.4 / Pattern 104 should have more than enough pins (in shell size 3), or maybe some of the Painton (small-pin) Jones connectors. The original Jones connector exists in 12-pin versions though...