How to tell if the rear element is wrong?

Hello everybody!
Recently I've bought a used Apo-Sironar-S 180/5.6.

Already on the ground glass I see heavy triangle-shaped coma from point light sources (does not matter are they in or out of focus) and quite fuzzy image anywhere but the very center of the image. Obviously something is wrong. I suspect the rear element is taken from another lens.

Is there a way to tell that for sure?

The front element has all the usual Rodenstock marking, but there is absolutely nothing on the rear one.

For example I can measure the focal length of the rear element alone, does anybody know this value?
Any hints are welcome

Thanks
Andrey

Return the lens.

Return the lens.