Congrats, I collect Holdsworth frame numbers and haven't seen this before, where we have what looks like a 70s frame (and most probably a Super Mistral) but the frame number doesn't make any sense in terms of known Holdsworth sequencing

One other thought, which doesn't actually get any closer than Allend has already, but if you knew whether tubing was 531 butted you could at least say definitively it was before 1982. The 1971 clubman had lug work like that, but the sloping fork crown and wrap around seat stay suggests a Super Mistral. Don't think you're going to get any closer than Allend has already. Very interesting though and great to see.