Adjustable front sight

[import-ant]

My dad has a s&w 586-1 .357 magnum revolver with an unfamiliar adjustable front sight that doesn't seem to adjust. It is like a platform with a dial which, when viewed from the top has four numbered holes with one hole appearing to be selected. It obviouslY adjusts the front sight but appears to offer only the four positions apparently selected by the dial.

1) what is this kind of adj sight called

2) how does it work?

Dial turns without resistance for short distance, maybe 1/8 inch or so in both directions, then stops dead. Selector on top of platform over the dial does not change numbers, nothing else moves. Is it broken?

Thank You !

 

[Scorch]

Pictures?

 

[drail]

It's a target sight for long range work. You have a click adjustment for different ranges. 50 100 150 and 200 meters. Yes, people actually shoot and hit targets at those ranges. Do a search for IMSHA competition. I would try to get it to seat at the lowest elevation and then adjust the rear sight until you get on target at 25 yards.

 

[import-ant]

The dial rolls a little to the left and right without resistance, then stops. Current setting is "3", won't change. Is there a trick to it? Is it supposed be hard? Currently shoots high...