No need to dig the book up because the exact quote is in my introduction to this thread.
First, Matt Jefferies gave a real life explanation
for the choice of numbering (probably also inspired by the registry number of his Waco airplane).
"Afterwards", he provided the in-universe
explanation. I read this "afterwards" as "right after" and not "decades later".
I don't see any design characteristics that would rather indicate this to be a design sketch for the Phase II
project. On the contrary, the "J" of the Jefferies signature is the "J" from the 1964 production sketches while the "J" of he Phase II
sketch from "6-77" on the opposite page in the Sketchbook is a simpler, newer version.
But regardless: If the creator / designer of the Enterprise
felt this starship to be the first of its kind, I consider him to be the ultimate authority on the subject - and not Greg Jein or Franz Joseph Schnaubelt.