Frame Numbers

[58bob]

Been looking through the knowledge base but cant seem to find anything. How do I identify a genuine '55 to '57 BT frame?
I've found stuff on the forging numbers on swing arm frames but nothing on the years I'm after. Couldnt find anything in Palmer III either. Thanks.

[RUBONE]

As has been discussed here and is in Palmer's, the straightleg frames from mid '54 through '57 all have a stamped build date on the top engine mount right side. It will give a two digit code (combination of letter and number) that identifies when it was made. No aftermarket frames have such a marking. The marking will look like the following picture.
Robbie