It could be because the uranium shows up things in glass better in photographs than it does in reality. I can see the letters clearly - and they look to be in the same "handwriting" - bit of an oddity for a fluke of the light.
It wasn't until I tried photographing a bird with an uranium streak, one day when the sun poked it's nose out for a few minutes, that I discovered a subtle blue streak in it as well. :pb:
Perhaps you should show your friend the photo, Craig - see if he can find the letters once he knows where they are. The L might be a Z, with the top bar of the letter unseen/absent.