What is the logic in the numbers that normally appear on the tail of a P-51? I was told that the first two digits were the year of manufacture, the the last four digits were the serial number. But there are Mustangs with 47----, and saw battle in WWII. So that can't be correct. Example here: What is the real answer as to the origin of the 6 digits on the tail?

The first number is the last number in the year the contract was issued. The rest is the aircraft's individual serial number issued by the AAF. In your example '44 is the year.

Thanks. Do you know the correct size of these numbers, in height? So if the first digit was a 3, that would represent 1943?