Below is a link to a current ebay auction selling a replica of Winston's gadget from the museum scene in GB2. It's built from a "Micronta 3001" metal detector from Radio Shack. After coming across this, I remembered that I had an old metal detector in the closet. Turns out that mine is the same model as the one in the auction.
Upon reviewing the scene showing the device, I believe that this MAY be the same make and model used in the film. If not, it's too close to argue about. Compare for yourselves and see what you think.
I'm in the process of converting it and will post photos soon. http://cgi.ebay.com/ghostbusters-metal- ... 1|294%3A50