...and now for an embarrassment of riches. Having cast around in several directions for rings, some more have shown up. They just arrived from Denmark this morning. They are in individual wrappers in a (really) old Hepolite ring box, the top rings are Vacrom and labelled as such, the box is labelled:
Original & Hepolite Pistons
1954/7, 646cc, O.H.V.
FH Huntmaster C.R. 7.25.1
70mm/2.7559" + 040"
and on the flap:
3650/#V + 040" ARIEL TYPE
I make no claims for their authenticity nor suitability. But you are welcome to them for the price I paid plus postage, say £25 in all. First to PM me gets the set.