Great Britain 1990 20p 150th Anniversary of the RSPCA, SG1480a.
A superb quality unmounted mint with full original gum bottom right hand corner marginal block of four, the lower horizontal pair with imperforate error.
An extremely scarce modern stamp error. Pierron records only 5 mint pairs in existence and especially attractive as part of a block of four. All errors occurred from the bottom row of the sheet.