Pretty sure you have a S&W, since there were no other manufacturers that used the type of cylinder catch that you described. The pin ahead of the rear sight is actually a spring loaded lever called the cylinder catch. The were used on 2nd and 3rd Models. If you depress the front of the lever, it allows you to screw off the cylinder for cleaning, etc. You must keep the lever depressed until the cylinder is totally removed or you will score the cylinder with the catch. Same with installing the cylinder back on the quill (center hollow pin) Look up under the rear of the cylinder and you will see how the catch retains the cylinder in place.

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Gary
SWCA 2515