This is the drawing from the
Official Guide To The Animated Series in reference to Carter Winston's ship in "The Survivor." It sport one nacelle rather than two. It somewhat resemblances what we actually saw briefly onscreen so matbe this was a conceptual drawing but not finalized.
I recall this design from the old
Star Trek Concordance, but I don't recall what episode it's from. I thought it was the survey ship Ariel, but I could be wrong.
I like the bottom image much better, but even so there is not a lot of detail to these ships leaving lots of room for interpretation and improvisation.