I've needed a 5C collet stop for a while and finally found a round toit in an old box under the bench...
This is a nice exercise in thread-cutting for the novice turner. The nut that fits into the collet is threaded 1.041-24 on the outside and 1/2-13 to match the same thread on the long rod. Obviously the lock nut is also threaded 1/2-13.
The long rod is reversible. It carries a 1/8" brass rod on one end to fit into smaller size collet openings and the blunt end is used on larger sizes.
It's a worthwhile addition to any turner's toolbox.