Good question.
First, keep in mind that determining motive, absent any clear statement from those doing whatever is being considered, is something of a guessing game.
That said, it is important to put this change in the context of the times. In 1965, the Wesson family divested itself of the S&W Company. It was sold to an international conglomerate, the Bangor Punta Corporation. Over the next few years, several changes took place in S&W revolvers that
I believe can be attributed to reducing production costs. The first was the elimination of all the flat latches (thumbpieces) on the J frame and KA frame guns. That happened in c. 1966.
Sometime in 1967, the phase-out of the smooth diamond at the screw location on both Magna and Target stocks began. By mid-1968, the diamonds were all gone. It is easy to see how this would be a positive step toward reducing both time and cost in the production process. I strongly tend toward the belief that those reasons account for the changes. If I'm wrong about that, it won't be the first time in my life!
If someone actually
knows that this is untrue and can document the correction of my belief, I would be happy to be set straight.