Dan. Hi Powers with shoulder stocks are rather expensive and collectable. If you become involved in it I would suggest having them appraised by someone who knows what they are doing same thing with the carbines. One simple proof mark can mean a lot of difference monetarily.
The other issue to consider is the legality of shoulder stock equipped pistols. Generally speaking they are an NFA controlled item meaning that he may have a federal tax stamp and BATF number assigned to the gun, if so the weapon could only be transferred to a class 3 dealer or someone who has completed the paperwork to go along with it.
Good luck and I am sorry for your loss.