TX700 frames were available as special order, with Columbus tubing instead of the specified Reynolds 531. However, this model did not appear in any of the Trek brochures on this site. Serial numbers beginning with M or N (41X and 61X bikes or frames) seem to have a different meaning for what normally is the year digit.
The year digit is the fourth character in the number.
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.
(Our thanks to Rich Ferguson, Fred Gomez, and Chris Tank.) The fourth characters are 2, 3, 4, and 5, there may be more.
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 has a leading B and has eight characters instead of the usual 7 (B4K6B109). Probably a TX200 with an extra character in the SN.
It has Sun Tour dropouts, and had a partial tubing sticker that included the text "TENSION BUTTED". Several frames with a leading serial letter of G have been reported with Columbus tubing stickers.Each seven-character number consists of three letters mixed with numerals.The serial number list and the code for the list are not available from Trek (but hope springs eternal).According to the brochures, Trek did not make 41X and 61X bikes or frames during 76, 77, 78, and 79 and also not in 83, 84, 85, 86, 87, 88, or 89.These serial numbers apparently do not follow the year convention used for other Trek models.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.