There’s two different versions of ESQ-1, a plastic and a metal one and they might be different. I just took a screw out of a plastic ESQ-1 (all we have in the shop at the moment) and it seems like it they used US screw sizes rather than metric. Seems to be a #4-40 and 7/16" long.
Most other Ensoniqs use metric M3 12mm screws, that should be what’s in your SQ-80.
I might be wrong on this but hope it gives you something to try.