This is the Beseler Anti-Newton Glass for the 45M, 45V and 810 Series Enlargers. This glass helps prevent "Newton Rings" which can appear as disturbing "noise" in your print. This phenomenon is a manifestation of a reflection between the glass surfaces of the negative carrier. Newton Rings appear as a series of concentric circles of both light and dark consistency.
In the Box
Beseler Anti-Newton Glass for the 45M, 45V and 810 Series Enlargers