Just remove the plastic cable guide to see the number.The other place Trek (or their subcontractors) stamped the SN is at the lower end of the seat tube.Model 950 frames were not made in 1975, 76, or 77, but only in 1980, 81, or 82, (according to the brochures).Another 950 frame was reported with a 0 (zero) in the year code, but was painted as a 1982.
These serial numbers apparently do not follow the year convention used for other Trek models.
Many early Trek bottom bracket shells have a one or two letter code stamped separately from the serial number.
This was made by the individual framebuilder to mark his/her work.
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.
By using the fourth character as an extension of the last three, the serial numbers could be extended from 2600 possible combinations to 26,000 for a given month. Serial numbers for four model 510 frames (with a leading F) have been reported that have similar unexpected year dates.