This FDC features the 3d Guernsey regional stamp (SG 10) on the front, and the Isle of Jethou 1962 Europa set (2v) on the reverse, both cancelled with “17 Sep 1962” postmarks.
The front of the cover, as shown, is illustrated with a picture of the Jethou coat of arms and a bird that appears to be a shag (similar to the design used on the island’s 1961 1s definitive stamp).
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.