doubleofive said:

The pilot Enterprise had a larger bridge module, larger deflector dish, solid red spiked nacelle caps, and holes in back of the nacelles. When they got picked up, they upgraded the model to have lighting, so they ended up changing all that. So there are really only two versions, they just used the pilot version in the series if they had a stock shot of it doing what they needed it to do that week.

They also used a few AMT model kits, but that's beside the point.

AFAIK, they only used the AMT kits for The Doomsday Machine to represent the wrecked USS Constellation. (The one exception might be the Enterprise as seen through the K-7 office windows in The Trouble With Tribbles.) Funny how scorching a model with a cricket lighter to simulate hull damage was a technique used well into the TNG era!