Another thought on the 17xx vs. 18xx numbers for Connies. IF any Connies can be said to have 18xx numbers, I would only allow xx ≤ 50, with 1850+ being Miranda/Anton/Reliant class vessels. New class demarcations for NCC numbers should be at a multiple of 10, maybe modulus 5 at worst case.
We know Jeffries said the 17 of 17xx was because it was the 17th design...that being how the ship looked in The Cage (no swirly lighting on the Bussards, antennas/probes affixed to same). All 17xx Connies therefore conform to THAT configuration at time of construction. From 2nd pilot (Man Trap) onwards, the revised (therefore, 18th) design, swirly Bussards, no antennae on domes, was the new refit. "Only 12 like it" could be construed as "refitted to the same level as Enterprise", with other Connies/Starships being in service but at the old (The Cage) refit level. Gooseneck-mounted monitors on the helm console, and all that. 18xx Connies, and possibly latter-half of 17xx, built new AFTER Enterprise's pre-Kirk refit, looked like Enterprise as seen under Kirk's command.
I attribute Excelsior's NX 2000 being "new" well after NCC numbers should be way higher (e.g. Entente, and the non-canonical Star Empire [Dreadnaught!]) to (a) the long-running nature of the Transwarp Project and (b) Kirk's reluctance to put back into port, lest he be promoted and taken out of Captain's chair. I'm figuring he brought Enterprise in on its absolute Scotty-jury-rigged last legs. As such, in my mind, Enterprise was one of the LAST Connies to get the "flat nacelle" refit. Starfleet knew that the flat nacelles could be problematic, but instead of tuning the warp coils properly (intermix) they dashed out, because extinction-level emergency, and as we saw in TMP, "wormholed".