Michael Blair wrote: "Here's a much better video on You Tube.  It clearly shows
the use of the mystery tool:

https://www.youtube.com/wa
tch?v=hI1jGwEfU8c


It's perfectly obvious now that I see it in use. In my mind I was making it much
more complicated than it is, with two independent screws instead of one and with
four cheeks not three, the central two spreading (I supposed) to leave a gap.
And I didn't realize that the cable draped around it on the wall was a part of
the tool. Thanks, Mike.

If I remember Clive Kilby's book correctly, English beer coopers would do the
same task by putting a big hoop on the spread-out end and then tilting it to
draw the staves in. Sounds simpler but its probably even harder work that way.
Tom Conroy