The KHT, introduced in 1938, was a refined generation upward from the KHM, in that it had a full cover over the ribbon spools instead of individual magazines, had a tab clear and tab set keys on the front instead of just the tab set on the keyboard, and was painted mostly in wrinkle-finish black.  It did still retain a tab clear lever on the back of the tabulator mechanism cover.  The margin release actuated when the tabulator was used, and that taxed the levers a bit--as was partly evidenced by the adjustments I have had to make on some of these machines.

Later in 1938, and running until early 1949, taking out for war production in 1943 and 1944, the KMM had its run.  It was an improvement in design over the KHT, in that the tabulator mechanism was improved, and the tab above it was gone. 

Sometime, perhaps in 1946 or 1947, changes such as square shift keys instead of round ones, a flattened carriage return lever stem instead of the earlier rounded one, black feet tops instead of chrome, and the use of aluminum shell casings instead of cast iron were seen.  Otherwise, they were still the same machine.

In early 1949 to about mid-1952, the KMG made its run.  Essentially, it was an extension of the KMM, but the machines were painted a gunmetal grey, all the keys were tombstone shape, and there was a quick platen removal mechanism on the right side of the carriage. 

This is the history as it was made known to me--through the typewriter database, and all of the Royal upright (standard) typewriters I've serviced in the last 35 or so years--and there have been many, as Royal was a very popular product.  From the pricings I've seen in the NOMDA "bluebooks," a Royal upright was cheaper to be had than say, an Underwood, or even more expensively, a Remington.  Royal made a very good product--rough, rugged, and reliable.  In fact, so reliable and solid were these machines that many have lasted well into the 21st century after performing many hours of long, hard work.  The mighty HH Royals that came after them, and the FPs that came still after, were just as hardy.

TypewriterKing, Thank you for the explanation! I never really knew what fine details differentiated the three models from each other until now. I really appreciated it!
Thanks again. OliverNo.9