I had one of the very first 1966 production formula S car’s, number 00521 of the assembly line built in September of 1965, and it came with a suregrip 3.23 ratio 7 1/4" diff. I Think that it could have come with a 8.75 if a box was ticked or if they weren’t trying to use up 7.25 sure grip units.
most of if not all later production S cars for the 66 year received 8.75" units