The 50-65 shim gives the most drop when the 65 is on top. However, keep in mind that you're not just lowering the comb, you're increasing the drop of the entire stock, changing the angle of the stock to the barrel, and changing the pitch. If just a lower comb is what you need, you need an adjustable comb.
There are a lot of gunsmiths that can do that, but TRON on this forum is the man.
Shims are available from Coles Gunsmithing. Google it. Also I believe Brownells has them. You can't get them from Beretta.
If you are right handed the 65 DX shim is what you will want for the most drop. 65 SX is for left handers. With the 65 facing you and at the top of the shim you will have it placed right before installing the stock. You can purchase the shim sets thru Brownells for about 10.00 plus shipping and that is where they get you. It is a flat rate. So small orders and small items get expensive fast.
I believe that the number on the shim corresponds to the amount of drop in millimeters. Thus a 65 would be 65mm of drop. I also believe that DX provides for "Cast-off", and SX is for "cast-on". I have heard, that if you don't have the shim between the buttstock and the receiver, set in the same orientation with the shim in the buttstock under the retaining nut, that you run the risk of cracking the buttstock.