I spend a bit of time looking at on line stamps and have noticed something. Some sellers list what they call signed stamps as if it magically makes them authentic if they happen to be rare. They could be authentication marks but I read somewhere that collectors tended to sign their stamps on the back as a mark of ownership. If I were selling a signed stamp for a significant price I think I would do some research on the mark. I assume most of the authentication marks can be found on line - I think I saw a web site somewhere.