This is a Fresnel Lens overlay for the ground glass focusing screen of an Arca-Swiss 6x9cm view camera.
A Fresnel lens is an accessory for a groundglass focusing screen. It helps "even-out" the brightness of the groundglass image and make the image appear brighter. The effect is more noticeable with wide angle lenses. Fresnel lenses are essential when using reflex housings.
Note: Current Arca-Swiss cameras come with Fresnel lenses as standard equipment.