If that is actually what you have, American collectors would call that Great Britain/Turkey Scott no. 6. It has a catalog value of $500 mint, $125 used, with the notation that these values are rough estimates because so few have been sold that establishing an actual value is difficult. There is also the important notation: "excellent forgeries are known". Forgeries are nearly worthless, so you would need to have your stamp expertized before you could sell it.
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