The Mercury crankshafts changed in 1951 to allow fit up of the torque converter for the automatic transmission. It was a 1CM prefix part due to that, but that may have changed if outside contractors ever got involved. The flathead Mercury crankshafts were made in some numbers for spare parts after the change to overhead valve in 1954 but I don't know if that may have been an identifier or not. With Ford, anything is possible.