Apart from the fact they are a malleable iron or steel casting the footrests would be difficult to manufacture due to the fine internal spline they have to allow for adjustment...That engages with a matching spline on the footrest support tube...
To accurately reproduce the cross hatch pattern of the military footrests from solid would also require access to a CNC mill...Ian
My intention would be to use the ones I have, reduce the footrest to fit inside the rubber then drill and tap the end for the bolt, is there any reason that this wouldn't work? Cheers Barry