Petersons use two different type of tubes.
The threaded, screw-in tube is known as a "chimney". They were originally made of bone, then plastic and I've also seen aluminum. If you sent a threaded stem back to Peterson, they will fit the tube. The tube is part of the system function. I assume many were discarded. Mark Irwin talked me into using the tube and it does make for a better smoke.
Here's a pre-Republic chimney, on 9BC Premier Selection pipe.
Here's the plastic chimney on a 1974 Shape 69
Peterson also used slip-fit, aluminum tubes on some of their lines (similar to a Duhill tube). This is a tube from Junior line pipe. The tenon, without the tube is very large and the draw too open, so I smoke this (and its stablemate) with the tube. The tube did keep the shank area very clean on a well-used estate pipe.