numbers/code question

When I have the "aircraft_(plan).txt. file open, the planes are listed by the number they're assigned, then another code number, then the description. Example;
AC#1,120,"Douglas DC-3 Paint 1"
What I'm wondernig is how is that number, such as 120 for the DC-3s, assigned? I'm wanting to add downloaded aircraft such as the 560 Twin Commander to my AI, but don't know how to determine what number to put into the code for the aircraft.txt file.
Does anyone know how to figure that out?
AC#1,120,"Douglas DC-3 Paint 1"
What I'm wondernig is how is that number, such as 120 for the DC-3s, assigned? I'm wanting to add downloaded aircraft such as the 560 Twin Commander to my AI, but don't know how to determine what number to put into the code for the aircraft.txt file.
Does anyone know how to figure that out?