coolride, If it is substantially lighter than the other revolvers you mention, I would suspect that something has been modified internally. I've owned quite a few S&W revolvers since the '60s. None had SA triggers so light as to cause me any concern. I have handled revolvers with internal modifications resulting in SAs so light, that if loaded, might have gone off if I'd sneezed....
Using my little Lyman digital gauge, I weighed the SA triggers of couple S&W revolvers, a Colt DA, and a Ruger DA. All break cleanly, none push off, and none feel unusually light or heavy to me. But I admit that I couldn't really tell you exact pull weights without a trigger gauge. The average was 4 1/8#, with a 686 being the heaviest at 4 3/4#, and a late '50s Colt 3-5-7 model lightest at 3 1/2#.
PS, fastbolt, a 686 was the last revolver I carried before we transitioned to semi-autos. Fine revolver, and with the L-frames, I routinely picked up a point or two in qualification over my scores with the Model 66
