Back when the 707 first entered production, Boeing came up with varied lengths for the fuselage to suit various customers. Later, the 727 and 737 series had the same fuselage cross section and cockpits as the 707. The similarities were noted, and a joke was made that Boeing had a machine that produced one long, continuous fuselage, and that varied lengths were cut off and made into different aircraft types as needed.
I don't QUITE think this trend has continued with the 757 and 737NG; though they are similar in fuselage circumference, there are some differences. Can anyone confirm this?