MINT- in Original Case!
This is a Topcon Incident Light Receptor that will allow you to used your in camera reflected light meter as an incident meter.
It slips right onto the front of most any Topcon or other brand lens with 49mm filter threads
to allow you to take incident readings right through the cameras viewfinder.
This particular Topcon item comes directly from the Leon Shoenfeld collection (who wrote the Topcon Book)
I believe there is a picture in his book that shows the two size Topcon Incident Light Receptors and this is the 51mm size that is pictured in the book.
Actual photos of item listed!