I have to admit that this fitting puzzled me the first time I saw it. The rifle had clearly been stripped a few times before and the one fastening looked as if it had a worn hex key recess. I spent ages trying to undo the 'hex head bolt' with (obviously) no success. So I gave it a gentle tap to try and locate the key a bit better ... hey ho, the central pin came out ... it came out with the opposite cup attached. Daft I know, no harm done except some embarrassment ... I felt a right twit but at least I solved it ...
Cheers, Phil