Have you read the thread http://goscf.com/t/27030? There's a lot of useful stuff in here, although at least one of the links to American Philatelist articles is dead. There should be enough in there for a bit of sofa-surfing research.
Thanks for replying. I did read through the article, I'll also check out the link. What Intrigued me most is that the top picture is the front of the stamp and the bottom picture is the back. Wondering if this has shown up anywhere else.