Ah I knew I shouldn't have said 'IBM PC 386'...! :-O
It was an "Epson Equity 386SX" which - as you say - is my uninformed shorthand for 'a 386-based PC compatible' (actually maybe it was only 286-based anyway?!)
https://theretroweb.com/motherboards/s/epson-equity-386sx-16...
ps: I was a mainframe PL/1 programmer at the time so plead ignorance on all things PC-based... (!)