I have a DobSTUFF truss DOB scope with an original Coulter 17.5" mirror in it and the star test is AWFUL! I should have taken a picture of it with my cell phone at the eyepiece but didn't think of it at the time and the scope is now stowed away with rain coming soon. However I would describe it as a deformed walnut. Kinda oval shape with the edges in and out. "Rings" is a term that doesn't apply.
The first thing I thought of was a bad secondary (it happens, see this thread) but once I looked at how the mirror was attached to the cell I think this is the source (see the attached photo). It looks like the mirror is glued to the arms of the cell with silicone or some such. If that stuff doesn't age evenly I would think there is a possibility of enough pushing and pulling between the multitude of attachment points to cause the problem.
However once I pry the mirror from the glue spots how do I retain it on the cell?