This may have been used on a military night vision or infrared binocular of some sort. Front clear diameter of 35mm (barrel is 43mm), rear clear diameter of 33mm (outer barrel is 47mm). The prism attached is a right angle prism with 6mm faces and a hypotenuse of 9mm. To the side of the prism is a 20mm opening (for light?), and underneath the prism (on the other side of the window) is what appears to be an optical filter. It is 9mm square and has 5 very small slits to allow light to pass. The barrel is 25mm long and stamped "0212" and "B&L 198F". Most are wrapped in the original packaging in mint condition. Limited supply so "First Come, First Served"!