All it would be is straight tubing that has been given a slight
tulip or trumpet shape for about 40% of its length.
Years ago I saw an exhibit at Goldsmiths in London that had bulges
stretched in tube as part of it’s construction. It said that this was
an old process rediscovered and was originally used to make the
hollow buttons that form the winder for large pocket watches.
I believe that this was done in the same way that the groove was put
in copper tube to form a hollow for the solder to sit in before ready
made fitting were available.
I have seen the tool that was used to do this it was a short rod
with a t-bar and a step in it’s size to only let it go a short
distance into the copper tube, it had a trapped ball bearing poking
out of the side, and a cam inside it. As you twisted the T-bar the
cam pushed the ball out to stretch a groove on the inside of the
I’d find a photo but I don’t know what it was called, the
girlfriend’s father who had one is long gone.
regards Tim Blades.