At the same place I bought another instrument which I saw.

It was called a speculum oris.

The dotted lines in the figure on the right hand of the screw, represent it when shut, the black lines when open.

It is opened, as at G H, by a screw below with a knob at the end of it.

This instrument is known among surgeons, having been invented to assist them in wrenching open the mouth as in the case of a locked jaw.