Mike - There was a marina blue 66 SS (painted orange) on eBay a couple months ago with untouched original white bucket seat headrest interior that had the S code. It didn't have any other interior oddities that might explain the S code as being anything other than the headrests.
I have also seen the S code after 763 on two other 66-67 coupes, one of which had headrests (claimed to be original), and the other of which had later model replacement seats, so no help. I have seen the S code after 763 on one 67 convertible basket case. The seats and almost everything else was long gone, but the bottom line of the cowl tag looked like a novel - had almost every option that could show up on that line. So it would not be at all surprising for that to have been a headrest car too.
I have never seen the S code on a 66-67 car that had its verified original interior that did NOT have headrests. But that doesn't really prove anything, of course. I have never seen the S on a 65.
I don't think the guage package or tach would have anything to do with the S code, as the dash was drilled, painted and installed (with the instrument cluster) on the final assembly plant side, not on the fisher body side where the cowl tag codes mattered. I don't have the lists in front of me right now, but I can't think of any option that was installed on the assembly plant side that ever shows up on the cowl tag (unless that option also required a body alteration, such as a 4-speed, or the M in group 4 on a Z16 which told fisher body to drill the rear trim holes differently).