Hi Steve,
From memory the rear sight was fixed to the barrel from the factory...but...BSA did do a scope arrestor that was indeed fixed to the cylinder and just in front of the back block.BSA scope grooves were quite shallow and if you didn't have the correct scope fixings things could move...and launching the .25 pellet might have made that worse?Maybe your's was drilled at the factory or later by a competent gunsmith...I did used to have a few of these curved metal arrestor blocks...
Value wise...i'd go £250 for your's....![]()