I'm told by at least one old Win 21/CSM 21 owner that there are numerous small differences on the CSM guns to improve the reliability of the 21 design. The one that comes to mind is the widening of the rib solder joints to improve strength and resistance to ribs coming loose. Frankly, in seeing a CSM gun next to an orig Win, the fit of the metal parts is clearly tighter and the finish of the metal to metal parts is better on the CSM guns.