This is a projector lens adapter. Right now it has a Nikon F-Mount connector.
This will let you use an older Bell & Howell 4" Trionar Anastigmat ƒ/3.5 projector lens on a digital camera. Just screw the lens into the adapter and twist to focus. Often produces very flat backgrounds but if you can get some point highlights just right it does some dramatic soap bubble bokeh.
By default the 4" Trionar is a macro lens on Nikon. Only focuses close up. If you need to have infinity focus cut the last inch (25mm) off the bakelite lens housing. This is to allow it to screw deeper into the adapter and focus to infinity with Nikon.
Includes an optional lens hood that can also hold a 52mm filter.
The author marked this model as their own original creation.