It started as a normal M3x0.5 socket-head cap, but I reduced the diameter and turned off the socket to fit the existing hole in the exterior floor plate:
The head was just barely too large for the largest of my pin vises. Drat!
The easiest way (for me, anyhow) to install that screw into the epoxy-loaded block started by dropping it into what seems to be a shim-punching tool:
It’s in the left hole of the top front row: talk about protective coloration, eh?
Then capture it in one of the Sherline’s drill chucks:
Which makes it trivially easy to turn right into the nut brazed to the floor plate and the epoxy inside the block. When the epoxy cures, the screw, nut, floor plate, spring, and block become one solid unit.
That punch block came with the lathe tooling, made for some special purpose long lost in history. It comes in handy all the time for other jobs, though, so I think it’s still happy.
(*) The pictures are staged recreations; I was cleaning off the bench and unearthed the spare screws.