Boeing also has gone a step further by incorporating customer IDs into model numbers: For example, Pan Am was assigned customer #21, United #22, Lufthansa #30, TWA # 31, Qantas #38, Varig #41, Singapore Airlines #12, etc...
So, when it's a Boeing model 707-131, we would know that it was a dash 100 ordered by TWA. A Boeing model B777-312 would identify it as a B777 dash 300 series ordered by SQ. Since there are more than 99 Boeing customers, the company had expanded its customer identification numbers into alpha numerics. "J9" for example was assigned to the former Imperial Iranian Air Force. A Boeing 747-2J9F would identify it as a dash 200 Freighter.