"Minor update. Mudd's quarters probably wouldn't work for Rand's future quarters in "Charlie X". In "Mudd's Women", there is a blue door and a ladder going up next to it (the same path Rand takes that runs into Charlie) which is not there in "Charlie X"."
I'm afraid we won't be able to find an optimal solution. Cabin # 3F 125 is also Spock's cabin on Main Deck 5. Relocate Janice's cabin from "Charlie X" there and you have major corridor upgrade rationalizations before "Amok Time" (and need to explain where Spock resided during "Charlie X"...
If we assume these used to be Mudd's quarters (also 3F 125) in the engineering hull we'll have these minor upgrades you mentioned.
Or we find a better way to decipher the code on these signs. My current assumption is this: 3 = three-room-cabin (office, bedroom, bathroom), F = section on this deck (could probably work if the bow section of the engineering hull begins with Section A and then progresses towards the stern), 125 = cabin number (while I have no problem with 125 cabins in the saucer, 125 in the engineering hull is way too much).
Maybe it's cabin # 25 and the "1" stands for something like first class. Compared to the cells of the brig Mudd's quarters in "Mudd's Women" would probably qualify as a first class cell.
P.S. One thing that's impossible to rationalize: There's a side corridor near the officers quarters where the bathroom / extra room has
to be (e.g. "Mirror, Mirror", "By Any Other Name").