James, thanks for those. Here are some of the others I have found, with thanks to the bike owners (where noted) for sharing the information.

XN > SLX NEW on a late Concorde Aquila... from fb user Steven C



SL > SL Concorde Prelude from 1994/5


GLX > Lemond , from fb user Doug H.


X > SLX Concorde Squadra


Y > PRX (very uncommon) Concorde Prelude bikeforums user sumgy


B possibly meaning SP, I'm not yet convinced this is Billato, I'd value an opinion either way. This is an eBay one