There are three "model" Ms, 75, 76, and 77, according to Cessna. The way I remember them is the 75M still had fuses, the 76M changed to circuit breakers, and the 77M changed to the the pre-select flap switch like the following 152s. The FAA only cares about the year the aircraft was built and the first '75 model M rolled off the assembly line in '74. Thus according to the FAA you could buy a '74L from early 1974 production or a '74M from late 1974 production after Cessna changed over to the next year's model. It's better in my opinion to go with the Cessna designation rather than the FAA's because it indicates what features to expect to find.
