After further looking, I was incorrect before in saying the SBE and SBE II have the same system, and now I remember the odd shaped plates on my SBE. So, in that case, there is only one plastic plate ahead of the stock, and then the appropriate metal plate. Yours is not set up correctly, and that plate cannot be reversed. They had actual left hand and right hand plates in that version.
Some of the descriptions on the site are incorrect as far as what SBE model they fit. But, just go by the pictures, and what yours has, and you will be fine.
Just to verify, is the shape of yours like this? Is this the set you found?
https://www.midwestgunworks.com/page/mgwi/prod/60161If so, here is the A S plate separately
https://www.midwestgunworks.com/page/mgwi/prod/60267 Anyhow, sounds like all you need to order is the A S plate separately and you should be fine. I wouldn't recommend using it with the right hand plate reversed.
On that site, if you search Benelli Stock plate, most results come up
That looks to be about the only stocking supplier, and their prices are the best anyhow
