The numbers are generated in order. They started with 0000-BBB, then 0001-BBB, and it'll continue until 9999-ZZZ. We're currently around MDB. This means that it's a good approximation to assume all the numbers are equally probable.
Yep, my parents purchased a car in 1978 that was still on "NNN AAA". Then our next car was a 1986 model, and I believe it was a "1 AAA NNN" series. By 1993, I purchased a car with ridiculous vanity plates, so I immediately pulled them off and registered normally, receiving a "3" series plate.