A perplexing philatelic puzzle

Philatelist and historian, Bob Ingraham of Canada poses a perplexing philatelic puzzle on his phenomenal website, Ephemeral Treasures.

Take a close look at the stamp (Scott #11) shown above. What appears to be a crease running down the right-hand side actually is something else entirely.

Bob got it in a trade on a tiny cover measuring only 11 X 6.5 cm. He says, "It was several months before I got around to looking closely at it. When I did, I realized that I had what collectors of anything call 'a find'. "

See if you can figure it out.

Give up? Click here for the answer.