Martin, You should try running an archive search here for anything with "Holdsworth", there might be a clue to dating yours by finding a close match to another serial number.
I have to check on MY Super Mistral (which supposedly is mid-70s, but I'm still not convinced) cause I can't remember its number right now.
here's one thread that contains a few pics and some mention of numbers:
there's more info than this in the archives, I'm sure.
EDIT: here's a quote that might help
"...31236. I'm almost certain it is a 1967 model..."
Last edited by unworthy1; 05-26-11 at 02:15 PM.