Greg's correct. Specifically, a cut square from a postal card (as opposed to a government stamped envelope), the writing on the back being the give-away.
George was correct.! I searched keyword ( KGVI Postcard ) and almost the same stamp come up on Google search with a 2-1/2d Value on the KGVI Postal Stationary card.They must have issued other values with the same KGVI Portrait.!
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And the reverse (Back side) added