I've always thought the design of the Grissom intentionally meant to look like a stable-mate of the Excelsior and therefore a modern design. Makes sense from a story telling POV as it's only logical that SF would send the most advanced science vessel available to them to survey Genesis. I suspect the low reg was inspired by the FJ numbering system where the scout classes get three digit reg while the heavy cruisers get four.
While at the time that made sense, the way things have been developed since means that is no longer the case.
Hence this: -
I really don't think anyone needs to find more of a reason to speculate. It's just for fun after all, nobody's going to change canon with this idea.