Surely this isn't actually a "proper" stamp? I think I've come across these before, but it looks like some sort of stamp substitute, hence it wouldn't be in the catalogues. Sorry I can't be more helpful.
It is a cut-to-shape portion of an envelope. Not the way to collect these. They should be either cut in the shape of a square with generous margins, or as the entire envelope.
Looks like a George V issue, but that's just a guess.
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