If you are referring to the spring in the hammer nose, not all K series revolvers have a spring in the hammer behind the nose. Usually reserved for the magnum caliber models, I believe.
Just to clarify - some do and some don't. I've even got doubles and triples of the same models of Revolvers and some have it - some don't. I never really looked in to when Smith had the revelation to remove them - because it simply does not seem better or worse weather it's there or not.
Sort of like the barrel pins.