Dating schwinn serial numbers
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.
On some bottom brackets there are two codes: one indicating the maker of the main triangle and the other for the maker of the rear triangle or perhaps the person who cut and shaped the tubing, if not done by the framebuilder.
If you know which mark was used by a particular craftsman, please let me know.
In the case of some early 80s Treks, the number is under the plastic cable guide on the bottom of the shell.
Note: This scheme is for all Chicago built non-Paramount models excluding 1962-1963 Superiors, 1964-1969 Super Sports and 1968 S/S Tourers (whose Serial Numbers are located on the left rear axle hanger and consist of a single letter code for the month [again, skip "I" and "O"], the last digit of the year and a 3-digit sequential build number -- e.g.
'K6018' is the 18th frame built in October of 1966).
This was used to keep the physical year of production, for example 'X9' -- December of 1979, with the model year of the remainder of the production for that model -- 1980.
This could also work for January back to a previous year.