Hi Kalookan, This is what the BSP has to say about the "A" notes..... “The serial number on Philippine banknotes consists of two (2) alpha and six numerics per denomination. The total number of banknotes per denomination that can be printed with SN A0000001 to SN ZZ1000000 is 600 million pieces. When the actual production of banknotes per denomination exceeds 600 million pieces, the suffix A is added after the year mark in order to avoid duplication of the serial number.” Why couldn't they just change the colour of the serial number from black to red or blue like they use to?