Are you referring to the little object swivelling on the tumbler? This is called a "Fly" and not a novel invention of the British gunmakers. My 1870 .500 BPE Rigby has tumblers with flies too. The fly is well known since flintlock times, being necessary for the functioning of any set-triggered lock. A fly keeps the unsupported sear from crashing into the half-cock notch when the hammer is on it's way down. A set-triggered rifle lock will not work without a fly! On locks without set triggers it allows a deeper half-cock or safety notch without risking a misfire in case the finger slips off the trigger.