The stock is an MDS stock bought on Ebay. The seller still have one available, it's only $125 plus shipping. Superb deal considering a rear stock from Benelli and ordered through Brownells will set you back $450. The ebay listings say that it will only work for SBEII but as you can see, it works with the supersport. the fore-end won't work though which is why i'm still using the carbonfiber one.
This is a very nice stock. You can still use the factory gel recoil pad on it. To fit it on the supersport, you just have to get a plastic spacer from benelli. the adjustable pad was installed by a local guy.
http://cgi.ebay.com/Benelli-Super-Black-Eagle-2-SBE-II-Shotgun-Stock-FANCY_W0QQitemZ200281329180QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2ea1b28e1c