Just out of curiosity I was wondering what the relationship was between the nose number that you see above the front landing gear, and the plane's registration? I though it was pretty straight-forward, but there are some things I don't understand. For instance, United's fleet of 757s are registered as N501UA through N598UA. The nose numbers are 5_01 through 5_98, with "_" being a number that represents a sub fleet (I think). The p.s. 757s have a "7" while the ETOPS 757s have "5". But then there is also 757s with a "4" and a "6: as the second digit in the nose number. What does that second digit represent?

And then the nose numbers on the 737s start with "99" or "94". And the 777s are even more confusing, the nose numbers start with "20", "23", "24", "27", "29". I can't seem to figure out the meaning behind the system, anyone know anything?