Nice looking sporter, appears to be a very high end professional conversion. I have dealt with those type swivels on other custom rifles. They work with a detent ball that is released when you press the center shaft in. In other words they are "quick detachable", problem seems to always be that they gum up over time and the ball will not release when the detent is pushed in. You will have to clean it very well and lube it in order to get the detent working again. PM me if you would like too.