Why is my 1d Black different?

Gustamps are often asked by customers why their 1d Black stamps are different from the one in the catalogue. The question to Gustamps often relates to the letters in the bottom corners of the stamp. Gustamps explain the different letters are a security measure. Gustamps added that the 1d Blacks are printed in sheets of 240 stamps. Gustamps point out that the first stamp in the top row has letters AA, the seconds stamp AB, the third stamp AC and so on . The second row are BA, then BB, then BC and the third row starts with the letter C. Gustamps make the point that it is possible to identify where a stamp comes from in a sheet of 240 by looking at the letters in the bottom corners. Many customers to Gustamps shop in Brighton like to buy 1d Blacks with their own initials on the stamp.