I have collected, bought and sold RR keys for over 30 years. There are 3 other marked knobby keys that I am aware of : P-RSL / PRSL (Pennsylvania Reading Seashore Lines), NYP&N (New York Philadelphia and Norfolk - very rare), and one marked OLE (I have seen 2-3 or these over the years and nobody knows what the letters stand for). I think the PRR keys had the knobby feature because they interacted with so many different RR’s, the switchman, brakeman, conductor, etc had to be able to tell the Pennsy key apart from the other keys on the ring, even in the dark, and the knobs sure do that, .
As far as the nickle-knobby key (which is solid, not plated), I was told that the plain key was for the Trainmaster, and the nickle key w. the wreath was for Division Superintendents ( I may have the job titles wrong, but you get the idea).