Actually, this wrench is not exactly new, but AFAIK it has not been discussed here before.
Disassembling the gas valve of a 391 is not something that needs to be done often (unless you use very dirty ammo), but when it does need doing it is a real chore with the little spanner wrenches Beretta provides. I previously reported on a special-purpose wrench called the "WonderSocket", but it is no longer available. A few months ago I stumbled across a wrench similar to the WonderSocket, but better in that it has a shaft to hold it steady and in place while the valve nut is being removed.
At left is the old WonderSocket on a 1/4" ratchet, at center is a 3/8" socket, and on the right is the new style wrench. Below the valve wrench is a 3/8" box wrench. The valve wrench can be used with any type of 3/8" wrench. I have used the box wrench to loosen the nut, then used the ratchet and 3/8" socket to completely remove the nut (it has a very long thread, and takes a lot of screwing to get it off). I think a ratcheting box-end wrench might serve both purposes, but I have not had a chance to try one. The only thing I don't like about this wrench is, it would be easier to use if it would attach directly to a 1/4" ratchet, like the old WonderSocket did.
As illustrated above, the wrench will work with either 12 gauge or 20 gauge guns, as they use some of the same valve parts.
I don't know who makes this wrench - perhaps one of you guys can tell us who does. I bought mine from a Gunsmith from NC named Mike Sherman, who operates as PMS Firearms. http://www.pmsfirearms.com/
He does not make them, but I am sure he could supply them. I paid $40 at his mobil shop last October. I don't know what he would charge to mail you one - his contact info is on the web site. I talked to a friend who is a retired machinist, and he said $40 for a part that complex is a very good price.