Well, like I said: I don't know (or much care, really) what the in-universe import is of the "BASIC SPECIFICATIONS, CONSTITUTION CLASS STAR SHIP" comment in the "Space Seed" script. I only point out that Greg Jein didn't simply fabricate the Constitution-class notion out of whole cloth by himself. I think there's value to knowing that it was actually scripted. way back in 1966.
My question earlier was simply if the remaining "exploded wing diagram" had "Consitution class" written on it and do we have a close-up of it to see (even if it is a "behind the scenes" shot)?
If no, then the Enterprise at that time could've been any class since we have only one on-screen indicator from that time frame, the "Starship Class" bridge sign.
If yes, then it is a clear case of the TOS Enterprise being a "Constitution-class" as that goes along with the dialogue of the story.
In either case, I think Matt Jefferies would've been at least in on the whole "Constitution-class" idea since he was the one who created those diagrams according to Tilotta's article. Since the "Starship Class" sign showed up on screen first, it would appear that "Constitution Class" was thought up as the first season progressed but no one got around to changing the bridge sign for the next two seasons...
Just to be clear, I'm not debating where Jein got his idea from.