My Holdsworth Special is an 80's fabrication having a full six digit serial number but with no leading zero (145344).

There's some "hash marks" preceding the serial # that possibly form the corners of some one- or two-digit code that could read sideways(?) if it is even an alpha-numeric designation at all. It doesn't quite look like the corners of a letter W or H, or even "11".
I almost thought it could be the frame size (23) reading sideways, but again the "dashes" don't really line up quite right for any of these possibilities.

I think it's a Marlboro-Era Holdsworth, likely the only one I've ever seen, either in pictures or in person.
Oddly enough for a presumed mid-to-late '80's bike, it has over-the-bb derailer cable guides, uses nutted calipers, and has long-style Campagnolo rear dropouts.

I bought it near-new from the original owner in late 1999, set up as a Rivendell-style build. I can't recall who/where he might have told me the source of the frame was, but it was described as a "touring" bike.
Seems to have a similar Reynolds 531C tubing sticker as my 1984 Trek 720.
After measuring it up I'd say it's between "road" and "sport-touring" (having 73x73-degree angles).

I rebuilt it (changed every part except for the steel headset) as a standard road bike, and it does have a most-smooth ride quality even with 23mm TriCompe tires on my narrow Aerohead wheelset. It ends up being essentially what I always wanted: a more sporting version of my "Cadillac" Trek 720!