Interesting.

I did not know this before.  Just found out that the sliding door window is not the same long as the driver side sliding middle window.  There is at least an inch difference.
This is an '87 model.
I don't remember this to have been discussed ever.  But I can be wrong.  
I did know, that earlier vans have different length of sliding windows to the newer ones though.  But I never thought the sizes may differ within one vehicle.  
Apparently GoWesty did not know it either.  But Van-Cafe did and have the right size of seals too.  
Zoltan