A well made extension tube for C-mount lenses. If you are buying this to extend close focus on C mount lenses used on a mirrorless camera, be sure to coat the rear threads and mating surface liberally with pencil lead. The large diameter of the lens means that the tube is screwed in with a lot of torque. When unscrewing, the tube stays in the C-mount to camera adaptor and the lens comes off. As C-mount camera adaptors are usually deeply recessed to allow infinty focus it can be very difficult to unscrew the tube without the graphite lubricant.
On a 75mm lens this tube reduced close focus to 75cm from 100cm, so you may need two tubes for extreme close work.