J3K7O90 - that is the number on mine - a 77 TX900. I was told that Appel brazed all of the top line bikes. I don't see another number or other marking anywhere on the bike...

I don't think we ever figured out what the last three characters in the serial number actually were, though...maybe that could be some code for the builder. I went through all of the serial numbers for the steel treks (the original builders logs), and didn't get any insight.