My Grandfather was a Cabinet Maker working for Western Electric for many years. My father was a Tool and Die Maker.
Together, they made a dozen or so parallel clamps, much like the ones shown. I have and still use them, today 60 or 70 years since they were made.
I see the nuts are made to swivel in the clamp to help adjust the clamp. That's a good thing. The only thing I wouild change would be to put one handle on one side and the other handle on the other side of the clamp. Much easier to adjust.

Bill