Ummm... you guys do know that the best way to match up with the details of those paintings is to start out with what they were based on... the 18" AMT Enterprise Model Kit.
I've worked with that kit enough in my lifetime to be able to pick many of the details out in a crowd. The primary hull and dorsal are from the original long box kit, while the dish is from the later short box editions. All of which shouldn't be surprising to anyone as there was an early study version done as a modification of an 18" model kit before work got started on the full size filming miniature. Of course the 18" study model wasn't fully completed as it was designed to help illustrate how the new major elements would go together, and so it wouldn't be odd for someone to assume that because the model kit originally came with a dish that one should be added to the painting (which is why the geometry of that dish matches the kit's version rather than the original's, and why the smaller dish from the later kit would be used rather than the larger dish of the original kit).
Some differences between the long box and short box kit included the dimples on the underside of the primary hull (which were on no filming model) are not symmetrical on the original kit but are symmetrical along the center line on the later kit. Also the dorsal was fitting to the contours of the primary hull on the earlier kit while the later kit had a flat area added to the primary hull's under side where the dorsal would connect.
The working sketches I've seen all indicate Jefferies planned to keep a classic TOS deflector dish. Why it isn't indicated on the finalized plans or schematics is a mystery.
Do you have images of these working sketches
by Jefferies? The concept sketches I've seen don't address details in that area and all additional drawings of the ship are not only missing the dish itself, but also any reference to the concentric rings behind the dish or where the dish would mount in the center. Even that part of the final model under construction is missing the rings and mounting (though there isn't any reason why they couldn't have been added in later*).
While I'm willing to admit that the absence of those details doesn't confirm that they were going to be absent on the finished model, I've seen no evidence (at all) beyond paintings (based on early test modifications of an AMT model... primary hull underside dimples and all) that it was going to be included.
And anything that would provide a hint (either way) of what Jefferies was actually planning would be a tremendous find.
* To answer the obvious comment of "but that part of the model isn't in any of the pictures,"
there is a nice shot from the front that shows that assembly sitting on the table in the background (next to what appears to be the hangar bay doors).