3. A set according to claim 1 wherein each banknote carries an additional legible code including, in reverse negative form, the same arrangement of alphanumeric characters as is included in the first-mentioned code upon a multi-color background different to that of said first-mentioned code.
4. A set according to claim 3 wherein in respect of each banknote the backgrounds of said first-mentioned and additional codes comprise different patterns of the same colors.