Slotted screw guide

[Toolmaker51]

The "special circle in hell" is because they didn't create hollow ground (parallel) bits to drive them with, and originally the slots were filed and or sawn (lacking mill cutters), which nothing worked short of hand-fitting. No question they were filed into an approximation of thickness but tapered, so it only bore on a slots edges.
A good case of which came first, chicken or egg.

Making very good parallel drivers is simple enough math; of bit size desired with rod size and cutter diameter for the offsets. Cut conventional, not climb, with smallest 4-flute endmill long enough for rod diameter. It's easier to index than compensate axis backlash, by repeating same conventional motion.
Do three passes; roughing, finishing after measuring, spring cut.
If it was good enough at the Weatherby South Gate Gun Shop in an Auto-5 Browning with crazy narrow but wide screw heads, well, it's good enough!
Use oil or water hardening drill rod, they'll last you forever once heat treated and drawn back. If it should break, cut off an inch and re-do.