Maybe you've already seen Norman Kilgariff's website devoted to Holdsworth/Holdsworthy, but here it is just in case:
http://homepage.ntlworld.com/nkilgariff/
He used to take serial number question emails, but has shut down that service due to a flood of them, I guess. Since he links to Yellow Jersey in Madison, WI, maybe they can be of service...or maybe you can get a clue from the catalogs scanned in Norman's site...good luck to ya!