What is the meaning of the letter "A" as a prefix of the KM numbers or pick numbers? Does it mean additional? Examples are the Philippines KM# A74 8 reales and pick number A1 for the 10 pesos fuertes (1852)

The "A" numbers in the Krause Mishler paper money catalogs refer to notes incorporated into their sequential system after the first numbers are used up.

In other words, if a note dated earlier than the oldest note they had previously numbered is documented, they give it an "A" sequence number. If I had to guess, "A" means "additional, "added," or "antecedent." I am not sure which.