Actually, other airlines sometimes have three or four numbers plus only one letter, instead of two. And there are frequent exceptions to the general rules: many Northwest aircraft's registrations end in "US," and Delta has some ending with "DN," etc. It's even possible to have just one or two numbers, plus up to two letters (like the corporate jet of a certain athletic shoe manufacturer, which is registered N1KE). But I digress.
Anyway, it's true that Continental is unique among the major airlines in using only registrations that consist entirely of numbers. I don't know why they do it, but there is some logic to it: although the first two digits seem pretty much random, the last three digits are the fleet number.
All I know is that the randomness of those first two digits makes it a bitch to keep track of the registrations when preparing financing documents for several dozen CO
aircraft at once, as I learned the hard way a few months ago...
Common sense is not so common. -Voltaire