Leitz 50X NPL Fluotar  oil immersion  Microscope Objective number 519-693.  It will fit the Leitz Laborlux series of  160mm TL microscopes.  The numerical aperture is (n.a.) 1.00 and it does not require a coverslip..  This objective works very well with other microscope makers from Nikon, Olympus and Zeiss as long as they are a 160mm TL version.  All glass is free from fungus, dust, scratches, element separation, or oil fouling of internal lenses. The image is very pristine, optically  just like new. Thanks for looking!