You posted a Guernsey stamp elsewhere.
Jersey and Guernsey are two bailiwicks in the English Channel, just off the coast of France. There are a number of islands in each bailiwick. The biggest and main islands are that of Guernsey and Jersey. Other better-known islands in the bailiwick of Guernsey are Sark, Herm, and Alderney.
The Channel Islands were occupied by Germany during World War II. The German administrations on the islands issued stamps. There were issues for Guernsey

and there were issues for Jersey.

After the liberation, the islands' postal services, again, were administered by the GPO in London. British stamps were valid for use in the Channel Islands.
In 1958, the GPO issued 'regional' stamps covering the most common tariffs. These were put on sale in the respective 'regions' instead of the ordinary Wildings for those tariffs and sold at the philatelic counter in London: Ulster (Northern Ireland), Scotland, Wales and Monmouthshire, Isle of Man, Guernsey, and Jersey. Again, there were different stamps for Guernsey and Jersey. They, however, are 'British' stamps. They were valid throughout the United Kingdom, the Isle of Man and the bailiwicks of Guernsey and Jersey. So, a Jersey 'regional' also would be valid in 'Guernsey', etc.
From 1969, the bailiwicks of Guernsey and Jersey ran their own postal services. (Isle of Man started in 1974.) These issued stamps that, only, were valid for use in the respective bailiwicks. In 1983, Guernsey's post office started to issue stamps inscribed 'Alderney.' These are like 'regional' issues. The stamps are valid throughout the Bailiwick of Guernsey.
Interestingly, the 'Machin' regional stamps for the Isle of Man remained valid in the UK until 31 July 2023, whereas they were no longer valid in the Isle of Man after its postal independence.