Put the gun in the NCIC database (report it lost or stolen), and with any luck, it might turn up somewhere down the road. FFL holders are supposed to keep entries and records, so if he was or is an FFL holder, there may be a record of where it went. This might also start a brief investigation by LE on your behalf.
Not trying to be a smart-aleck but have you tried Walther directly? If Fort Smith can't help you then surely they can get you in touch with someone in Germany at the factory who is a historian, expert, etc.. Dr. Peter Dallhammer, Product Manager of Walther Arms, from the History of Walther would be a good one to ask (see link below). Maybe you could get in touch with him. Probably doesn't get any better than a letter from him authenticating what it is.