I'm a bit late getting back myself. Life happening! I'm guessing the addition of ebone forend tip and deluxe four screw-base swivels, 'probably occurred sometime in mid 1992 or later. Yet there are other possibilities too. First, the black tip and swivel sets, may not have occurred together in the same time frame. A possibility. Also, it might have been that at some point, the "Super Grade" designation was limited to the higher grades of wood.
I have no answer to these possible issues. The only sources would the the factor whose response seems to be silence and of course quite possibly records of how many of any particular model not itself qualifying as a separate serialization series (ie other that "G" prefix), might not have been historically preserved whatsoever. Difficult to know. Considering collectors in general don't even have a precise year of manufacture tied to serial numbers, I'd not be optimistic about any factory information several decades old forthcoming.
What we do know is that sometime, 'almost certainly' in the 1992-93 period, Winchester decided to up their stock decoration under the "Super Grade" designation. Notably the original pre '64 Supergrades had good quality, but not necessarily fancy grade wood stocks. The early Model 70 copied the Model 54 Supergrade stock externally about exactly, including the Super Grade black tip & SG swivels.
I've enjoyed this effort to document something more of Winchester history. A pity parent FN isn't more forthcoming. Perhaps they feel as we build reverence for earlier guns, it works against the pushing new guns out the door. Who knows!
Best & Stay Safe!