I tried a Google search for Iron Sight Gun Works with no luck (I'm guessing they're long defunct by now), and the Gun Parts Corp. schematics don't show the ghost-ring sight model. I can order a detent ball from Brownells...if I know the size.
Does anyone happen have an 870/1187P with these sights that could measure the detent ball and it's spring? I was able to rig an elevation tension spring by clipping a few coils from an old hammer strut spring from a 3rd Gen. S&W in order to have the sight maintain it's elevation adjustment...but without the detent, the screw just turns with recoil and the sight adjusts itself.
Thanks!