Noticed on some of the older kit guns there are 2 pins in the frame above the hammer and under the rear sight. I know one retains the firing pin. What was the other for? And when and why was it eliminated?

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S&W Accumulator

The other retains the elevation stud that the elevation screw works on.

The J Frame elevation really is a screw that works against the stud in the frame that has a rib on its' face, and there are two grooves on the bottom of the screw. Together the grooves and rib provide the clicks. The larger frame elevation "screw" is really a nut that works on a stud that pulls the sight leaf down against its' spring tension. The J elevation is a jack screw that lifts the leaf against spring tension that pulls the leaf down. Same operation, completely different way of achieving the same result.

The pin was eliminated by changing the design of the stud. Instead of having a shank, it is now just a disk that is staked into a recess in the frame.