I'll toss in a couple more names; Holdsworth and Bob Jackson made 531 mixtes
and ask a trivia question:
What did the builders use on 531 mixtes to make the long "top tubes". Reynolds didn't draw special extra-long thin tubes of 531 just for mixtes, AFAIK.
So, is the set always a mix (with a couple non-531 tubes thrown in) or is the construction always a "hybrid mixte style" with a regular single down tube with a 2 more stays attached, or did they splice stays together to make the long tubes?
That Claude Butler (Holdsworthy, no doubt) sure seems to have a 531 decal on the forks, if nothing else.
edit: just looked at the eBay, pics, the decal says 531 frame tubes, so maybe that indicates a mix to include the 2 long ones made of something plain.