rfaspen
[insert pithy phrase here]
Guesstimating from the image in the OP with a ruler included, this fits the scenario. I'm guessing the microscope objective in the OP is a rather old one (perhaps as old as 1920s or 30s?), but appears to be RMS.
This is a Leitz microscope lens, mounted in an M42 lens cap for some kind of photomicrography that I could not get to work. Microscope objectives are tiny little things. And yes, I think that it is an incomplete lens.
Canon F-1 Microphotography hack by Mike Novak, on Flickr
