I found this image of the Nydar 47 sight. From the research into the mounting system on this device it looks like it takes several screws to mount it properly! (Minimum of two screws up to six).
I don't think one screw would do the trick. I am leaning more toward the notion that it was a bead mounted sight that Tom talked about. I don't think I will ever know for sure but it has been fun figuring it out.
BTW, you can still purchase the Nydar sights in very good condition on eBay for those folks that like to collect different related shotgun paraphernalia.
Thanks
Mike