My guess is that a prototype, being one of a kind, is worth whatever someone is willing to pay for it. Just gotta find that someone. Gosh, that sounds like a smart remark - surely not meant that way. The only source of some expertise that I can think of off the top of my head, is an outfit in the Quad Cities that sells on consignment and auctions off many rare and unique collectors' items. I believe it is called the Quad City Gun Auction. You might try a Google search for them, or if you don't find them, I'll dig into it and get some contact information for you. They are well known here where I live and most of my gun friends feel that they get pretty good prices for sellers - if that is what you might do. Let me know if you need help getting in touch with them, if you are interested in doing so. Maybe they can at least help you with some info.
