So this is what a Miranda-class from Star Trek was supposed to look like during The Original Series era

The USS Cortez from Star Trek: The Early Voyages issues #5-6.

The Early Voyages was a series published by Marvel Comics back in the late 90s, which chronicled the adventures of the USS Enterprise under Captain Christopher Pike's command. It was meant to fill-in the gap between the original Star Trek pilot, The Cage, and James Kirk's tenure as Captain. The Cortez's design is supposed to be a pre-refit Miranda-class, the same class that the Reliant from The Wrath of Khan belongs to. The two-parter featuring the Cortez is even a clever reference to that movie. In the comics, the Cortez is captured by a splinter society of Vulcans who never embraced logic like the others and when the Enterprise shows up to find out what happened to the ship, the latter is attacked and almost destroyed by her.

I think there was a lot of speculation in the Trek community after The Wrath of Khan about whether the Miranda-class was in service during The Original Series and if so, what did it look like? Unfortunately, the comics - like everything else except the TV shows and movies - aren't canon, so it's a moot point. That's probably the one thing I don't like about the Trek franchise, Paramount's canon policy..

(Picture via Memory Beta, the non-canon Star Trek Wiki)