Originally Posted by
PAXboy
Welcome aboard FlyingGoggles. In contracted form this became 721 and 722. Which is why the 747-400 is the 744. ... Again, an Airbus 340-600 is a 346.
The contracted form "744 / 346" is necessary for airlines computer reservations system which allow a maximum of three characters for aircraft type. In the early days everything was shared via teletype in which every character cost money to send.
Now that we have modern internet systems the characters are still limited to three, because it's too expensive to change the system and maintain a standard, for no return. Most airline employees have these memorised.
This is also why IATA three letter airport/city codes are used vs. ICAO's four, and why fare basis codes are no longer than 12 characters.
P.S. The "SP" in 747-SP stands for Special Performance. Boeing took the standard 747, shortened it, gave it bigger wings (and tail) so it could fly higher and further. I believe PA introduced it nonstop on the JFK-TYO route which was a remarkable achievement in those days. Iran Air was another early adopter flying it nonstop from Tehran to JFK, in the days of the Shah. They still fly this model, but not this route, one of the few airlines to do so.