Nice! Thanks.
I seem to remember seeing M frames listed. I think those were the tiny ones favored by hookers and other sneaky characters. I could be mistaken.
The I frame was the predecessor to the J frame. The .32 Hand Ejector was the pinnacle of the I frame, IMHO ... an almost perfectly proportioned revolver.
The M frame was a tiny .22 revolver from the early 1900's. It was called the "Lady Smith" (not to be confused with the LadySmith J frames that came around in the 1980's) and was chambered in .22 Long (*not* .22 LR). More than a few M frames have broken forcing cones because someone stuffed a .22 LR round in one.
The X frame is the really big guy ... chambered in .460 S&W Magnum or .50 Magnum. I believe the X frame shares some internals with the N frame, but I could be mistaken.
There were other letter designations for some of the really early stuff (top break revolvers), but I don't recall those off the top of my head.