Hi Jamn 44,
The C series M&Ps were S&Ws third million starting in 1948 (pre 10 5 screw) through the pre 10 4 screw 1955-1957, Model 10 in 1957, Model 10-1 Heavy barrel 1959, in 1961 model 10-2 std barrel and 10-3 heavy barrel Changed the direction of thread on ejector rod and on the heavy barrel front sight from 1/10 to 1/8", In 1962 10-4 lost the trigger guard screw on std bbl, the 10-5 the front sight went from 1/10" to 1/8" on the std barrel, also in 1962 the 10-6 heavy barrel lost the trigger guard screw. the D series came in 1967 and was the 4th million.

It looks like yours is a Pre 10 5 screw. :mrgreen: