I've curiously watched the development of the Moxon twin screw vise over the past year or so. Curious because while the current iteration copies the twin screws, the chops are now made of comparatively thinner material.
When you look at the original engraving, which in turn was taken from Felibien who in turn most likely took it from, etc. etc., the chops are robust.
It's always difficult to interpret old engravings. Perspective, if at all present, was often ignored or simply unknown to the engraver/artist. Proportions could be the equivalent of Gulliver's Travels on a bad day.
Even so, the twin screw vise shown in Moxon, which did not carry through to Nicholson or Martin, resembles the Bookbinders press. The square section chop seems to me to be a sensible form for a vise. Instead of a tall and thinner chop, a short but squared profile would prevent any chance of deflection, dampen vibration, provide ample room for clamping to a bench and lots of depth for threading.
Moxon describes the workbench in his inimitable English ( Download Moxon workbench,1) but only briefly discusses the twin screw vise as something to be clamped to the top or side of the bench. Felibien shows an enormously out of proportion twin screw vise behind his bench, which I can only assume meant he was in need of a visit to the Optometrist.
Someday I may be moved to make a twin screw vise if and when I find a need for one. Until then, I hope that someone else has fervently copied the original Moxon pattern and can report on it's functionality.
Till next, Gary