Carl Zeiss Stereo Microscope Lens Attachment
A quality attachment, but I am unable to identify its exact function.  Please let me know if you have expert knowledge.
The rotation is smooth and it is in very good cosmetic condition.  The optics have some visible marks as shown under bright LED light - please see last two photos.
Measurements:The diameter of the attachment ring is 52mmThe diameter of the lens is 37mmThe two circular holes are 15mm across.