The silver knob ring contains the DOF marks. And they always have to be in the same position to the rest of the barrel.
If that ring was to set the aperture (and not for max. twist as for closing it) it would be in various positions.
Thanks all. The mount is dedicated. It is attached with screws. I had a Soligor T Mount years ago that used adapters for various camera mounts that could be changed easily.