I've only ever seen 7 of these and 6 were mine.

My Mercury link, below, shows one by Nick Howes and one by me:

http://philippugh.fortunecity.com/Philip%20Pugh%27s%20Mercury.html

Although it is quite elusive, I've managed to see it most years. All you can really ever see is the phase.

However, I find it hard to believe that there aren't other photos out there somewhere.