Bausch & Lomb CAT. NO.53-70-29. Made to fit the B&L 53-70-28 Stereo Rhomboid Arm. Inside is a coated cemented doublet lens with a focal length of 620.4mm, a coated double concave lens with a focal length of -55mm, and an elliptical front surface aluminized mirror, major axis is 34mm and minor access is 24mm. 4.5mm 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.