Murray McL -

Were you enamored of the old capos, or responding to JfromEB?

I'd suggest trying the ancient product before discarding the Shubbs (or whatever other modern instrument of instrument torture one might favor).

The "string and bar" devices, with peg torquers, quite probably were well suited to old gut-strung instruments, and should be good for modern flamenco ones; but the higher tension of metal strings may not play well with their best features.

The string direction is at an "adverse vector" direction to apply much pressure in the direction needed against the stings/fret without rather high tension in the wind-up string. You likely can wind the peg up tight enough, but the device may suffer "wind damage."

If they work for you, then obvously the proper course is to make one (or buy one if you must) and use it. I'd lay the modern version aside where recovery is possible until testing is well advanced though.

John