It is likely these frames, SNs beginning with M or N (Models 41x and 61X), were contracted out or were made in a separate Trek facility, and were given the old serial number form so as not to interfere with the sequential numbers being assigned by Trek in their main shop, which began in late 1980.
This variation to the SN system may have been done to extend the numbering system.
"Rub the area with candle wax of a color that contrasts with your paint. Note that even with the serial number filled with wax, the leading "0" is almost imperceptible."Before late 1980, Trek used an alphanumeric serial number scheme.
In the meantime, we have the results of the serial number decoding project:). This letter may also have been used for custom 750 frames.