You need two matching shims. The one you show, and a matching one that goes in the butt stock under the draw bolt. It is simpler than it looks at first. The SX and DX stand for Sinestra and Destra, left and right in Italian respectively, and the number is the amount of drop in millimeters.
This equates to "cast off" for right handers where the stock is slightly to the right of the centerline of the action (DX) and "cast on" where it is slightly to the left or (SX). So, the one you show in the picture would give you a 55 mm drop with cast off for a right hander, or if you install it the other way it will give you 60mm drop with cast on for a left handed shooter. To keep from cracking the stock you need the corresponding washer in the butt stock.
Check Cole Gunsmithing
for the shims. Cole's is pretty much the authority on all things Beretta. Call them.
MGW may have what you are looking for, but I would call them.
There used to be some third party folks that made shims with more and less adjustment than the factory ones, but I don't know if they are still available or not.