I just picked up a vintage Trek and am having a hard time determining frame age. Serial # is not standard (5B6C51). The only thing I can figure is that I am reading it wrong due to a lite stamp and it may be (E5B6C51) ((first letter may not have made impression on steel)). This would be a Feb. 1976 frame. This seems unlikely as I have not heard of a frme older than Sept of 1976. Furthermore, the frame has the headbage glued on and not screwed and all components match the 1980 412. In addition, the hubs are stamped 8/80.

Specs:
Serial#: 5B6C51
Size: 25"
Tubing: Ishwata 022
Wheels: Rigida 27" blue label with gyromaster hubs (rear procompe freewheel)

Any thoughts.

I am assuming it is a 1980 412 with a messed up serial #.
Only other explanation is a very early 1976 frame built up to 412 specs in 1980.

Thanks