Thanks for your thoughts, Michael.
I think this unique CAS picture confirms for Jan that her cavum is indeed cavum
Unique since page 9, the Introduction in Richard Hamblyn’s book Extraordinary Clouds, Met Office, David and Charles, 2009, says ‘Take the image on page 73 for example, which shows the unusual sight of a fallstreak hole illuminated by the prismatic brightness of a circumzenithal arc. It was taken by the Cambridge-based photographer John Deed who came across the sight by chance while taking his son to football practice one cold September morning: “While he was warming up to play I went for a short walk. When I looked up above me there was this large tongue-shaped hole in the cloud. This was interesting in itself but then I realized that there was a rainbow at one end or, as I now know, a CZA. And as I looked closer there was a second one further down, although it does not show up on the picture very well.” A few days later, this rare – possibly unique – capture of a fallstreak hole with a double CZA is donated to the CAS’s photo-archive, and something entirely new was added to the global repertoire of clouds.’
