I don't know. My knowledge stems from my biology classes at school and what my parents told me. And that was some time ago. :) I didn't know the word cere, so I looked it up and found the UV part about owls and felt the urge to share it. I didn't look into that study until now.

I'd like to know more too, but I don't think I have the energy to pull through if I'd dive into that rabbit hole. So I won't try. :)

The authors state: Rods are responsible for the gray scale scotopic vision under low light conditions - so grey scale brightness, only. The cones are not relevant because it is too dark. Or are they? There are those oily cones which maybe can detect UV light? If so: still no colors, just brightness. To me it looks like the authors of the study offered some educated guesses.

I would say it means: Either it is greyscale brightness but maybe it is something else. But the authors don't know. They just know that the owl perceived the difference.

It looks like the owl's eyeball doesn't absorb the upper range of UV-light (350nm+) so it could reach the owls receptor cells: https://journals.biologists.com/jeb/article/224/20/jeb243129/272645/Lens-and-cornea-limit-UV-vision-of-birds-a

I didn't find anything about that secondary (beta) peak that I could understand - maybe it is about the biochemistry in the rod/cone which can be split into phases when light hits the cell. I'm lost here. The google-LLM pointed me here: https://pmc.ncbi.nlm.nih.gov/articles/PMC3265844/ I'm not even sure, if this is a match for our context.