Some questions about aircraft series-numbers:

If an airline, for example SAS, should sell or lease some of their Boeing aircrafts to an another airline what would happen to the series numer? Would it still be 7x7-x83 or would the other airline have to put their numer instead? As far as I know the last two numbers is the number that the customer has at Boeing and if an aircraft is sold from an airline to another, Boeing has nothing to do with it, have they?

Are all numbers (from 01-99) in use? I have seen that some airlines has a letter and a number. After all, that make sense, because Boeing must have more than 99 customers.

Has Airbus the same system as Boeing to "Identify" their customers?