B&W 41 mm UV (403) filter - Not to be confused with UV-Blocking filters that photographers normally refer to as "UV" filters, this one passes UV A radiation (320-385 nm), but blocks visible light and looks pitch-black to our eyes.
Used with appropriately sensitized films in such applications as ultraviolet reflection photography in forensics or in materials research, but also as filters on UV-emitters for fluorescence photography
Depending on the illumination and on the film's sensitivity, it's filter factor is in the range of 8-20