Ooooooh, the cutter! Little too focused on the work; typical for me.
But DIYSwede has explanation of what is uber plausible. That taper and finish marks fit right in with hand generated cutter movement.
Printable View
Exactly. Prior to this frame you can see the stock is a rough (probably forged) piece of rectangular stock. The arrow points to the square part. He rough turned it because finish is not important. Since it is a short, tapered length, he probably hand-turned it with the compound rest set at the angle. Under those conditions it is difficult to have a smooth, continuous turning motion as you stop to reposition your hand on the handle to complete another turn. Each pause will leave a "spring" mark. If he used a taper attachment then it was set up lose and backlash (or feed rate) is the cause of the rings. Since the pitch of the rings is so consistent I suspect it may have been power driven in some manner.
Attachment 41995
Now off to find a piece of thick wall large diameter pipe!!!! Or, maybe start with a smaller one. Come to think of it, I think I may have a piece of 4" schedule 40.....