If it's the Dufferin Phantom (what they used to call the sneaky pete) it has the pin on the shaft, and the insert in the butt. It's a 5/16-14 thread pin. Some of the sneakies came with aluminum inserts. Some had brass. It will fit a standard 5/16-14 if you run a tap through the threads in the insert. Or...better yet, get a repair guy to drill out the insert in the butt, dowel the hole, and install a pin in the butt.
Otherwise, have him retap the butt, and a standard pin will work fine.
Edit: just checked one of my Dufferins. It's the Grenadier, which uses the same pin as your sneaky, but opposite configuration. The butt has the pin, and the shaft has the aluminum insert. The pin is a standard 5/16-14. A Joss shaft fit on a standard 5/16-14 pin, and the Dufferin shaft fit on a standard Joss and Jacoby 5/16-14 pin. If you'd like pictures for comparison sake, let me know.