What is this strange mirror lensboard?

[schneidk]

Found this weird lensboard with a mirror on the back. Any idea what this is?

162949

[Bob Salomon]

Probably part of a mirror alignment set for copy work and for seeing if the standards of a view camera are properly aligned or an enlarger is properly aligned. They were made by Linhof, Hasselblad, Zig a Line and others. Properly made ones use front surface mirrors, one at the subject and one on the lens. Pointed at each other. When you look through the gg or the finder you will see an infinity of black holes. The idea is to adjust the camera or enlarger until you only see one hole. Using these you can adjust parallism to tighter then camera manufacturer's tolerances, the Zig measures to 1/5000".