All of the Kodak 120 film I shoot (Plus-X, TMX, TMY, E100 and Portra) has these numbers. They are alternate frame numbers - if you shoot 6x4.5, they line up correctly with the frames. If you shoot 6x9, you get two numbers per frame.
I don't have any idea why they chose 41-55, but it certainly makes it easier to specify negative numbers to your photofinisher.
IIRC they have done this for as long as I have shot 120 - since the 1970s.
I don't have any 220 negatives handy, but I think they do the same or a similar thing with them as well.
Matt is correct, I remember when they first started doing that. There was a Kodak publication that stated so.