Also a possibility that TOS was trying to emulate the stellar types of O-B-A-F-G-K-M with a similar classification for planets.
Why O-B-A-F-G-K-M? Why that order? Why seven letters?
What does each stand for?
Apply those same answers to "Class M" planet, I suppose.
Maybe TOS planets were classified T-Y-F-K-M-O-P or any other assorted arrangement.