Had to come back to this discussion with a link. https://ia902600.us.archive.org/27/items/starlog_magazine-017/017_text.pdf

When they decided to green light a movie in 1976, Paramount had just rejected Gene's treatment for a Trek movie that he had submitted, "The God Thing" (yes, he had to pitch an idea just like everyone else, as he had sold his rights to Trek away years before), and they picked up a treatment by Chris Bryant and Alan Scott; what we know as "Planet of the Titans". McQuarrie was getting a reputation in Hollywood as the "go to guy" for SF concept art in the mid 70s, so it's no surprise that Paramount hired him to work on Star Trek.

The production designer on "Planet of the Titans" was Ken Adam. That drawing was one of his modified versions of the Enterprise based on McQuarrie's preliminary painting. They built the study model from that. Since Adam was the production designer, it means that if "Planet of the Titans" had gone into production, we would have seen an Enterprise very close to that.

Paramount had decided to move production of a Star Trek movie to the UK to cut costs, and I believe they hired Adam and Derek Meddings, as they had worked on recent Bond films, for street cred. Bryant and Scott submitted their first draft of the script in 3/77 and it was rejected in April.

Shortly thereafter, Paramount announced a new TV series because they didn't have faith in a Star Trek movie, then "Star Wars" and "Close Encounters of the Third Kind" were released within months of that, then Paramount killed the series, and green lit another movie in January of 1978.