Have you tried talc and the uv light
together to see if anything is revealed?

I was hoping the mark might be in the concave bit of the pontil mark.
Although perhaps cinnamon and the torch might be better. Talc probably has "brightening" rubbish in it.
Marks can be found in odd places. I once found one on the bottom part of the handle of jug.
I can't remember if it was Stuart or Webb-C. But it was a well-known one - in a very odd and unexpected place.