Up for your consideration is this Carl Zeiss Jena Tessar 50mm f/2.8 Zebra Lens-M42 mount with a currently open aperture.  

Please inspect the photos closely, what you see in the pictures is exactly what you get! 

Here is the deal on this lens.  It has a bad hold open spring for the aperture blades.  The aperture blades still open and close smoothly but the spring that operates this mechanism has lost it's tension and will need to be replaced. 

The good news is that the glass is clear, no fungus scratches or damage.  Focus has a slight resistance in one direction so I'm guessing it will need to be re-greased but you can still use it right now.  

So you can still use this lens wide open at aperture F2.8 although it would probably only be good for a mirrorless camera with adapter. 

Feel free to make an offer and thanks for stopping by!