OPNLguy From , joined Dec 1969, posts, RR: Reply 2, posted (9 years 3 months 1 week 7 hours ago) and read 1377 times:
If by "extensions" you mean the listings in various aircraft databases like, say, a 737-2H4(A), the "A" designates the "ADVanced" version of the 737-200 (Southwest, in this case) versus the earlier "Basic" version of the 737-200.
Regarding the 737s, the ADV models started showing up in the early 1970s and had several improvements over the Basic models.
-The engine forward engine cowlings were re-designed, and no longer had the little "sucker doors" that opened at high power settings.
-The leading edge Krueger flaps were extended and ran closer to the fuselage, resulting in better takeoff performance.
-Structural weight limits were increased. For example, max landing weight went from 98,000 lbs on a Basic to your choice of 103,000, 105,000 or 107,000 depending upon whether your order JT8D -9, -15, or -17 engines, respectively.
The 727-200 family went through similar changes...
To clarify, the fuselage of the 737-100 was stretched and that became the 737-200 (Basic) and the -200 Basic could indeed carry more passengers than a -100. The changeover from -200 Basic to the -200 ADV in 1971 kept the same physical dimensions, IIRC, and any increases in pax capacity between the -200 basic and the -200 ADV were coincidental as to what a particular airline ordered as an option on their -200 ADV's, and wasn't a function of the changeover itself.
As an aside, the 737-200 Basic with -7 engines was a dog aircraft that couldn't get out of its own way on hot days, and/or at hot-and-high airports.