The Western aircraft that you pictured is an older -200 "Basic" version" as evidenced by the "sucker doors" on the engine cowlings, and the leading edge Krueger flaps (between engine and fuselage) that stop short of going all the way to the fuselage.
Boeing started making all 737-200s as "Advanced" (ADV) versions in 1971 or so, and the ADVs had numerous improvements over the Basics. The sucker doors disappeared, the engine cowlings got a little longer, and the leading edge Krueger flaps now went much futher towards the fuselage. As a result of all these aerodynamic improvements, permissible weights also increased.
I'll have to do some checking (with some retirees, since the Basic/ADV changeover was so long ago) to be sure, but I seem to recall that the #2, #3, #4 and #5 slats on the ADVs were re-engineered to deploy further in the "full extend" regime, thus matching the #1 and #6 slats (the most outboard). Looking at http://www.b737.org.uk/history.htm
and scrolling down to the section on the ADVs, they mention the changes in slat sequencing as well.
I'll try to track someone down and post an update later...
[Edited 2004-08-01 23:50:12]
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