There is a rail that clamps on where the half-moon cut outs are. I have not seen any but talked to one person who had tried the base and was unsatisfied with it. His complaint was the base would not stay attached to the receiver.
As I had no first hand experience, I cannot say whether it was operator error, a design flaw with the mount, or a poor idea overall. To me the design reminds me of the rear adjustable Redfield style scope mount and that design did not have a great reputation with heavier recoiling rifles. But, Ruger uses a similar concept with their rifles so who knows.
I'd be willing to bet typing something like "Beretta 391 scope base" into a search engine would get one some examples. Whether any are worth the money would take some research.
