reply to u/Greydusk1324 about the difference in Royal Standard typewriters at https://reddit.com/r/typewriters/comments/1skmfum/comparing_royal_standard_desk_machines/
I'm (sorry?) to report that the internals of the Royal standard typewriters including the Ten, H, KH, KHM, KMM, KMG, RP, HH, FP, Empress, 440, 660, etc. are all incredibly similar if not exactly the same over several decades. The biggest change is probably the introduction of Magic Margins with the KMM. The margin release button also moved down to the keyboard around this time as well.
Most of the rest are smaller, subtle differences in how the ribbon reverse mechanism is done or things like keytops changing from glass and acetate to plastic, the threading design of the ribbon vibrator, as well as the external design and some of the other small fit and finish. Some of the much later models allow one to remove the entire chassis from the body of the typewriter to make cleaning and servicing easier.
There are certainly differences in type-feel and "weight" in the changes in the keytops, but broadly they're all mostly the same machine. The biggest differences between them all (for me) tend to be how well they've been maintained and/or been cleaned and adjusted. One seriously well adjusted Royal is better than any 20 other random Royal standards you might pick up for a fraction of the price. Of course, if you're doing your own wrenching work, then once you've learned one machine well, the rest are a breeze to work on and bring up to snuff.
If you think there's a huge difference between your KMM and KMG (which are probably the two closest models), then perhaps it's worth it to try some others? The biggest difference may be the FP which has chunkier key caps that have more effect on the "feel". The HH and many of the other later models have thinner key tops. The Ten is probably the most different from the rest. The H, KH, KHM are what I would call "experimental" models moving toward the perfection in the KMM and KMG.
Context: I'm an owner of a KHM, 2 KMMs (including a 47+ pound, 18" wide carriage), 2 KMGs, 2 HHs, half a dozen FPs (in all the colors but Willow Green), and a 440. This includes a variety of their standard pica and elites, a Clarion Gothic, and a Pica Double Gothic. Stylistically I love the KMG and the FP, but my KHM has one of the most satisfying "actions" of any of the machines I own.
Of course, all this depends on what sort of collection you're aiming for. I love a good Royal and have a smattering of other makes and models, but I am slowly working toward a completist picture of Royal Standards. I do try to add machines that have a unique typeface or other feature as I add more of them to get some additional depth and breadth to my collection.
If you're a collector with limited space, then pick up the best looking design(s) (for your personal aesthetic) and rest easy that you're not missing too much. You can also pick up new machines to curate for a few years and then move them along to other collectors to enjoy so that your collection is always changing.
You might get some more detail and nuance by watching Joe Van Cleave's YouTube channel where he's done a few dozen videos on Royal standards as well as comparison videos over the past several years.
Good luck on your hunt!