@ MGagen & Shaw
Many thanks for the details and clarifications, which literally put things into their proper perspectives.
Now it rather appears that Matt Jefferies did not
like the VFX hangar model. Although the VFX model had only pairs of two windows in the observation corridors, Mr. Jefferies obviously preferred to have pairs of three.
It's also obvious that the engineering section is slightly ahead of the (yellow) bottom circle.
I just can't help but wonder where this now puts the engine room(s) of the TOS Enterprise
According to "The Immunity Syndrome" and the alien entity's exit point in "Day of the Dove" (TOS without R) it would be ahead in the engineering hull's bow, according to the Phase II closeup detail it would be astern.
Good thing, then, that I visualized one engine room in each of these locations in my TOS deck plans to be on the safe side.