Upload & Sell: On
A spring-loaded detent pin is different from the stop screws on the RRS plate Steve shows above. Those stop screws are designed to catch in relief grooves cut into the top of the clamp.
The pin is also a safety feature and is designed to catch in the cutouts you see in the plate bottom in the pic. Many don't like the safety pin, I do and have used them for years but have always bought Markins clamps that have these pins. You could probably custom-machine an older clamp for a pin but it's probably less expensive to buy a new clamp that's got one fitted already.