I just ordered two Williams 5D-GR receiver sights for a couple of .22s that I have. I think there is a piece missing on both sights. There are two allen screws oposite the angled part of the base that is supposed to fit into the dovetailed receiver. I thought there is supposed to be another sliding piece that can be screwed down from the top slot and from the allen screws. Anyone have any ideas if I'm missing something?