Need some help.  Looking for a rear sight for a Hastings Rifled Slug Barrel.  The model that I have is a "paradox" and is for a Rem 870.  I contacted Hastings and they could not help as they stated they only have sights for their current production barrels, which is different from the paradox design.  I have the base and the dovetail mount but not the adjustable part for the rear sight.  Any recommendations?