B&W 77mm UV (403) Extra Wide Glass 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.
B&W Extra-Wides are special filters for wide-angle lenses. In order to prevent vignetting, the accessory thread on the front is larger
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