Glutenboy, while I probably had a few slices in the 70s, my passion for eating pizza didn't really bloom until the 80s, but, as far as my fixation on ingredients and conversing with staff goes, that wasn't until the 90s, and, by that time, it was all HG, so I can't say when the migration occurred with any real certainty.
To complicate things even further, my two favorite pizzerias from high school (80s), although still open, have thoroughly debased their crusts, and, much like you, I'm working with a 30 year old memory. I can picture an image of a slice like it was yesterday, but, the texture is a little tougher to nail. I know it was very puffy- and very white- extremely white. Snow white with a few charred areas. It was characteristically chewy, but I don't think it was ever leathery. I've made AP pies that have approximated it, as well as All Trumps doughs that were close too, although, in all fairness, the All Trumps dough was barely kneaded. Out of the possibility of my place using either an 11-13% protein dough versus a barely kneaded 14ish dough, my money is on 11-13. 13%, though, was still part of that AP/HG migration.
I think Peter's done the most digging into the AP/HG migration history, so I defer to his timeline. For me, though, that was before my time.
Even though my favorite place debased their crust over time, fortunately, for the last 30 years, they haven't changed either their cheese or their sauce. I can say, with absolute certainty that they've been using Grande all this time. Now, as far as Grande's past penetration into the market, I think it's safe to say that they most likely have garnered the market share for at least that long.
Stringy cheese, in my experience, is undermelted/not bubbled cheese, and undermelted cheese rarely oils off or produces the precious orange drip. Straight from the oven the cheese would be very melty, and if you picked it up too soon, perhaps you could get a pull away strand of cheese, but, by the time you ate it, it was very cohesive and toothsome. The best description I can come up with is slightly pebbly textured and a little spongey.