Rob,

The shape of the two tools is very similar but but that's about all they have in common.
In the case of the mystery tool it clamps to a bench, or table, where a pipe die, generally, the pipe is clamped and the die is rotated. If we look at the clamp on the mystery tool it doesn't look all that substantial to me, which leads me to think that not a lot of weight was swung off the tool in use. If it was clamped to the bench it would be on the front edge, the work can't protrude through more than a couple of inches and if anything of length was held in the jaws then you'd need another support behind you?
Don't worry, when I first saw the photos I went straight to the thread cutting tools section of my McPhersons catalogue in the hope of finding it there.


Geoff