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...
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