The Virtual Machin Album

The key stamp of the gravure high value set is the £2 stamp with the missing pound sign. When the stamps were intially released, no one noticed that one stamp in half of the panes was missing the pound sign. Fortunately for collectors, it was the stamp adjacent to the cylinder number, creating lovely pairs, strips and blocks with the cylinder number in the margin.

For more details about how this error occurred, see the news story about these stamps.