"Well, I think you would agree that "BASIC SPECIFICATIONS, CONSTITUTION CLASS STAR SHIP" and "BASIC PROPULSION SYSTEMS, CONSTITUTION CLASS STAR SHIP" in the "Space Seed" script probably transcend "conjectural fan speculation."
Looks like we are stuck in an orbit like Enterprise
in ST II.
Picture yourself to be back in 1966 and on the set. The writers guide has established the Enterprise
to be a member of the Starship Class, and if anybody has forgotten a quick glance at the dedication plaque of the bridge set will read "STARSHIP CLASS".
Then comes a script talking about a "Constitution Class Star Ship". Now, the actors and producers may remember that it had been established in "Tomorrow Is Yesterday" that the Enterprise
has twelve sister ships but everybody will ask "What the heck is a Constitution Class Star Ship?"
If Khan is supposed to be looking at basic schematics of the Enterprise why
doesn't the screenplay just say so? Anything else - from a production's point of view - is redundant or confusing or trivial technobabble.
To cut this long story short - again - the Making of Star Trek
from 1968 makes no reference whatsoever to a "Constitution Class" but instead explicitly to the well known "Starship Class" and an "Enterprise Class". To me that's the ultimate account of what the producers / creators obviously intended.
Apparently, the cosmic consciousness is not without a sense of humor:
If I understand correctly, the first space shuttle built was supposed to be named Constitution
but partially because of Trek intervention the name was changed to Enterprise
"In-universe", however, we seem to have a mirrored situation where many prefer the first ship not
to be the Enterprise
but rather the Constitution.