I noticed decades ago that early (1948-50) K-frames have a different shaped hammer. Not all of that vintage have it but, I have never seen it later than 1950. I have a K-22, K-38, M&P and Masterpiece with this hammer. I call it the Superman hammer because if you look at the opening sequence of the original Superman TV series the gun being fired is an M&P with this hammer. All my guns are under 100K sn. I have never seen any reference to this different shaped hammer (spur), I would guess that it was just one of those design changes that happen but, I am curious.