Jim, thanks very much for the reply. I'm more puzzled now than I was before.
As I look at the picture of your rig, I see a speciman holder, not a device that indicates where the plane of best focus is and that shows the area the film will see.
When I think of a focusing frame, I see two rather different devices.
One is a copy stand for a screw mount Leica. These stands have four legs. When the stand is placed on the subject, usually a document to be copied, the legs' feet define the rectangle that the film will see. And when the camera, with the right lens and diopter mounted, is laid in the copy stand the subject will be in good focus.
The other is a rectangular wire frame somehow held in front of, e.g., an Instamatic with a diopter lens attached. The frame is in the plane of best focus, makes it and the area the film will see visible. In practice, one holds the camera so that the frame surrounds the subject, e.g., a flower and pushes the button.
I'm sorry to be so obtuse. What am I missing?
Thanks again, regards,
Dan