Boat Buzz

Replacing the exhaust elbow on a 1940 Model B. Having a machine shop remove the water fitting (shown in picture) from a used elbow that I bought. I will need to replace the fitting. Is that some kind of a compression fitting that will squeeze the copper tube? Anyone know where I can get that fitting?

I have seen these nuts available on http://www.classicboatconnection.com
These are basically a nut that compresses a rope gasket wrapped around the overflow tube. When you get it apart you will see how it works.
I use wood stove door seal seated in hi temp RTV for the gasketing.