I believe that you will find that the early production 55 tractors did not have as many Letters involved with the serial numbers. For example, my 55 diesel western is truly a Western/wide axle with the arched front axle and 23.1-26 rear tires/wheels, but its serial number is simply 55 DSH 1309. Up until sometime during 1952 model year, each of the vaious configurations had their own set of serial numbers. During 1952 Model year build, Massey switched over to serial numbers starting with 10,001, and from then forward all the different configurations used that serial number series. At that time they also added the additional letter/number designations, including fuel type, standard or high altitude, chassis configuration, and clutch type.
This is based on a combination of what I have observed and what is listed in Keith Oltrogge's Massey Tractor Data Book. If any of you have concrete data and/or examples that prove this wrong, we would all love to see it!
I am also hopeful that somebody out there has a 1953 M-H 55 Western that will share their full serial number with us, so that we can see what letters were included used.
Thanks,
JB