In the 50 M and N serial numbers that have been sent in by owners, the year numbers go from 0 through 9.
The remaining part of the serial numbers seem normal.
This does not apply to the all-numeric serial number form that began in late 1980. For these all-numeric numbers with a leading zero, see LATE 1980 to LATE 1986 below. It appears that some custom frames were built that did not fit within the standard model description. The last three characters - The final letter ranges from A to Z and the final two numbers go from 01 to 99. Perhaps once Trek was nearing more than 2600 bikes/frames of a given model per month, a new serial number scheme needed to be developed.
At some time late in each year (beginning generally September or October) the frames produced were created from designs, and painted with colors, for the next year. In this case I used a red candle to make the serial number stand out.
"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.
The dates coded in the serial numbers, or the dates inferred from the numbers, are dates on which the new frames were stamped with serial numbers (serialized).
These dates are not necessarily the same as the model year of the frame. I have included a photo of the results from my bike.