(9/21/2020 8:32:05 PM)
RE:33 Diesel/Kerosene registry
I believe you will find that somewhere around 1953 when Massey started using 4 digits instead of two to describe the model (i.e. D1RF vs. just DR, or D1SF vs. just DS), they also started adding the P to the end of the diesel serial number to denote it was equipped with PSB fuel injection pump.
As for the 5 digit serial number in SD, Keith Oltrogge's data book shows Model 33 serial numbers in 1955 from 9782 - 12607. So, 12501 would be very near the end of 33 production. That same data book states: "Although no actual production figures exist by engine type, some former Massey employees report that only 150 diesel versions were made."
I guess that was always good enough for me. ;-) (Where is that emoji button??)