Agreed. Initially, I thought that it might be due to the fact that Shimano did not have a SIS compatible freewheel that was larger than 28T, which is what the GS & SGS versions would have been used with. However, Shimano upgraded Deore XT to SIS in 1987 and the RD-L525GS & SGS still weren't listed as SIS compatible. Furthermore, Shimano introduced a new Light Series in 1987, the L532. Again, the SS version was SIS compatible, but the GS & SGS versions weren't! I can only assume that there was some sort of difference in geometry, The upper pivot casting would have a longer drop and/or different angle to accommodate the larger cogs and there may have been some other differences associated with this casting. It certainly would be interesting to try a long cage Light Action rear derailleur with SIS shifters and SIS freewheel.

Regardless, I second the numerous comments that the Light Action series shifted very well. However, they do have an Achilles Heel from a durability perspective. The cut-out in the parallelogram's rear arm to accommodate the pre-select mechanism does create a weak point and I have seen a number with broken rear arms.