Pictured is a ausJena GF Plan Apocromat 50X Reflected Light Microscope Objective Infinity corrected. It has an n.a. of 0.90 and is intended to be used on Infinity corrected reflected light microscopes from ausJena, probably the Carl Zeiss Jena Amplival research unit  The coverslip information is: 0-C. Since the objective can be removed from the dovetail assembly, I would assume that the objective can also be used in a transmitted light mode. The objective thread diameter is 28mm and the dovetail spacing is about 50mm. It is in excellent cosmetic and optical condition. Thanks for looking!