Salvador 500/O351a

     The design on the stamp was first used in 1924 (Scott no. 500). It shows a map of Central America with the country of El Salvador shaded darker. Also portions of Mexico, Northa and South America appear in the design. In 1925 the same design was overprinted Oficial. The mis-spelling "Atlant co" appears on both issues (500a, O351a)
     I have not found any description of the cause of the error, but since it appears on both issues and because there are stamps without the error (500, O351), I believe that is more likely to be a production error than a design error.