This is an ORIGINAL Mainspring (also called hammer spring for a SET TRIGGER on 1873, 1892 and 1894 (pre-64) and model 53 Winchester lever action rifles. Spring that puts tension on the hammer when cocked. This mainspring has an off-set narrow top and is attached by Screw wp47 as used 1892 and 1894, and model 53, while part 621A is the mainspring screw 1873 model.