Don is correct ,it has a value to me and would purchase it for a few dollars .
How is the price determined ,it is determined by the forgeries that are up in public stamp auctions ,I can usually buy them at a price between $2.00 and $4.00 a piece and get them by the hundreds ,there are two lots right now up for auction and the price is estimated in the stated price range .
But back to your stamp .......first thing always is to buy a few yes a few of the regular stamp and compare them to what you got inverted ,it saves a lot of money and if need be come on this chat room and have someone like me or many others to put up a regular stamp and wait for the comments .
Here is a real stamp and maybe one of the regulars here can capture both your and mine and put them side by side for a real look at what the experts in Britain were looking at .
