Hmm, I think I grasp the problem now. I tried pressing on the rivet with a pen whilst the turret was in the "groove" and it did sink down a bit as well as holding the turret down for a few ratchets. It popped back up just as quickly though. I gather that at the other end of this rivet is something that should be bedding it in and that is the loose part?

Oh well, the next part will have to stay on hold until I can get a soldering iron. I'm determined to get this guy fixed. Thanks for the help once again.