The rear sight base is tapered to the taper of the barrel and held for and aft by a cross-pin and located against rotation by dual purpose screw. The hole you can see if for the nose of this screw. The barrel came fitted with the sights as a matched set. I haven't ever successfully matched up an odd rearsight bed with an odd barrel yet. Scraped the interior of the backsight bed until it locks onto the barrel but never managed to do that AND align the existing cross-pin holes.

No doubt others on this forum will have........ but I ain't. I have secured them using other means (and that's NOT locktite or superglue either.....!)