e-RICHIE asked noted: the original gig was rsf, no!?
Well my first Carlsbad bike was A57 1S9. fob Carlsbad, in 1973
That was pretty darn early.
MCXX for size came after that, it was not confirmed until 1975 that the odd serial number & signifiers on that frame declared it was of the first batch.
John Jorgensen
Palos Verdes Ca