Interesting question, though not, by any means, restricted to amplifiers (see cars, vaccuum cleaners, etc).
A little search on the etymology of the phrase didn't yield a direct answer, but here's a guess based on what I saw.
One archaic meaning of "mark" is boundary. "Mk II" would suggest an expanded boundary (presumably for performance) of the "Mk II" device in question, relative to the MKI. Similarly, the "Mk III" would push the boundary beyond the "Mk II".
Just a guess.
Marty
A little search on the etymology of the phrase didn't yield a direct answer, but here's a guess based on what I saw.
One archaic meaning of "mark" is boundary. "Mk II" would suggest an expanded boundary (presumably for performance) of the "Mk II" device in question, relative to the MKI. Similarly, the "Mk III" would push the boundary beyond the "Mk II".
Just a guess.
Marty