Bausch & Lomb CAT. NO.53-70-31. Made to fit the B&L 53-70-28 Stereo Rhomboid Arm. Inside is a coated cemented doublet lens with a focal length of 52.4mm, and an elliptical front surface aluminized mirror, major axis is 34mm and minor access is 24mm. 5.5mm thick. Mirror material is optical glass, Index of 1.517 and Abbe V# 64.5. Top has bayonet type mount with a 25mm opening. Of course, if you have one of the B&L Model 240 Aerial Photo Interpretation Zoom Stereoscopes, then you definitely need these.