I’m pretty sure that’s the design for the enterprise from Star Trek Phase II, a series that never got made; this design was then developed further into the now familiar refit from TMP
I knew about Phase II, and that elements of it were merged into TMP and TNG, but I only remembered the triangular Enterprise concept that was later the basis of Discovery.
This all sent me down a rabbit hole where I learned things like Xon (originally a replacement for Spock) became Saavik, and Decker and Ilia were the basis for Riker and Troi.