The one in that link is missing the other piece of hardware that came with them when they were sold new.
It was a threaded insert that you installed in the wood. That was already threaded inside for that allen stud that you see on the back side of the cap.
The cap then screwed down onto the flat surface of the grip tightly.
You adjusted the timing or clocking of the cap to come out right by small adj to the allen screw stud.
IIRC the instructions recommended a coating of shellac on the final instal to seal the cap in place.
You could turn these down very tightly and tap them to final position with a small block of wood w/o damaging anything. They didn't come loose. But could still be removed the same way if needed.
Here's one with the instruct and insert.
https://www.gunbroker.com/item/792515648
