I'm imagining a scenario in which someone with an old Carlton road frame with Capella lugs, possibly with a damaged rear triangle, walks into Ken Bird's some time around the mid-70's asking for the rear triangle to be either replaced or shortened, with track ends and fastback seat cluster. Possibly the bracket was replaced at the same time, which would explain the non-Carltonesque frame number. That would be a lot of work, but perhaps cheaper than a new frame.
I mention this as a possibility, because if I had got my hands on an old 531db frame in the mid-70's, one with unfashionably long chainstays, large clearance, lots of fork rake, etc, and had the funds, I would have contemplated doing the same thing, schoolboy fashion-victim that I was!
Judging from the reminiscences, even if you didn't have the funds, cleaning Ken's car a few times might get you a discount.
Presumably Alec Bird would have done the work, and the frame resprayed and Bird transfers attached.
I suppose the only way to verify this theory is to strip the paint around the brazed joints and check for a disparity in brazing style between the headlugs and the seat cluster/bracket etc., which would probably prove inconclusive anyway, so who wants to do that?
Just another theory-feel free to shoot it down!