Explanation of A after year on Philippine banknotes

Over the past year, several Philippine banknotes have been issued bearing the letter A after the year. I recently received the following explanation from a source at Bangko Sentral ng Pilipinas:

“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.”