Re:
Let it be understood that I have no 'inside information', and what follows is based mainly on pics/information gleaned from the Kilgariff site, possibly misinterpreted by me, and bikes seen on this site:
The 'Holdsworth Special' seems to have been introduced some time around 1975/6. The early ones had a distinctive lugset/top eye treatment which after a few years seems to have been superseded by Prugnat longpoints/minimal 'semi' wraparound. Spookily, two examples of this early 'Special' have appeared recently on this forum with consecutive frame numbers, despite one of the owners maintaining that his frame was 'early seventies', even though it had the later serif decals afaik not used before 1976. (this frame number is included in your 'sample', dwscrimshaw.) Those numbers were in the mid 41,000s. I realise that this is right at the late edge of your temporal area of interest, but, pending a source of inside information coming forward with words to the effect: "No, we used that lugset/top eyes all through the '70s" , This evidence suggests to me that you might have mis-attributed frames that should be 'around 1976' to somewhere around 1974? If correct, this is also evidence that the new numbering system did not start until at least around 1976/7.
In case you missed it, 'the other one' is here:
viewtopic.php?f=22&t=347031
I might be wrong, and this might be some standard lugset from the '70s that passed beneath my radar- always glad to be informed/corrected...