Europa 1961 imperforate miniature sheet from Jethou, featuring a block of four 6d stamps.
U/M / Unmounted Mint / MNH / Mint Never Hinged.
Part of the Bailiwick of Guernsey, Jethou is a small island, roughly half a mile long, that sits immediately south of Herm. It is owned by the Crown, but leased by a tenant.
Between 1960 and 1969, a period when the island was open to the public, Jethou issued its own local stamps. These were seemingly produced mainly to generate interest and publicity, though postally used examples, affixed to the back of mail from the island, do exist.
Islands within the Bailiwick were no longer allowed to produce their own local stamps once Guernsey gained postal independence in 1969, meaning that Jethou's issues - like those from Herm - ceased.
Further background about Jethou stamps can be found on this website.