I can't make out any eye structures but I'm no eye specialist I must admit. Are the specialists you've spoken to from a palaeontological background? Maybe it would be helpful for a mineralogist to have a look. Sorry I cant help any more than that. I'm very interested to see how this ID pans out. Good luck
Many fossil animals had prismatic eyes, which (being mineral) could be fossilized. It is rare to find them as cross-sections (which yours appears to be). I find it strange that the surface is as flat as it is, but maybe there was a vein there, conveniently. Also, ammonoids were thought to have complex eyes rather than simple prismatic ones.