Memory serves, they used to be a series of numbers and letters.
What do you mean by “Locator” code?
suranyi: The “record locator” is the six-character code that is issued to identify a particular booking from departure to final destination. Looks like DCMFQV or B1PA21 on the Boarding Pass, ticket, and itinerary. It is distinct from the ticket number, which is unique for each leg of the trip each way for each person.
SanDiegoTim**: I just looked up one of my travel accounts, and since Spring of 2008 it shows fourteen (14) record locators, of which half ( 7), the latest one in August 2009, have at least one and often two and once up to three numeric characters — most often on on USAirways. So it may be a matter of your usual choice of airlines or booking sites.
The codes though in my experience, at least since back when I had to use them often at the job I did in '88-'93, have always leaned more towards letters – if using all basic ASCII numbers and letters, each place in the code string can have 26 possible letter characters vs. 10 possible digit characters. I would not be surprised if they have rules of what character(s) may go in what place in the string (specially first and last).
Just a guess: confusion between O and 0 and 1,I and l