As long as we're talking about these things, I'll add another one of their intended design functions.....they are intentionally desisigned with flat sides, so that after you have the piece that you are working on clamped in its jaws, you can then position and clamp the flat sides of the hand-vise in the jaws of your bench or post vise. That way you can use both hands to file, thread, or whatever else you are trying to accomplish with both hands for more control and accuracy in your work. I actually use mine a lot more that way than I ever do trying to 'freehand' anything with it.
There are modern reproductions of these things available on Amazon. Be aware that there are two different types available......one like hush's and one with a fancy turned handle meant more for the jewelry trade. They both work for 'free hand' work, but you can't clamp the one with the wooden handle in your bench vise without damaging the handle.