When note collecting began to spring up in 1961 (officially) a man named Albert Pick becan to catalogue all the different notes and organize them. The first notes issued by an issuer were P-1. 'P' for Albert Pick, shortened to just Pick and the '1' was the cronological order. There are however some exceptions to this rule.

FROM WIKIPEDIA
Albert Pick (born 15 May 1922, Cologne) is a retired German numismatist. An internationally acknowledged specialist author on the subject of paper money, Pick wrote the first modern catalog of banknotes in 1974, and is widely credited with establishing the modern face of banknote collecting. His Standard Catalog of World Paper Money is the standard reference work for banknote collectors worldwide.

Pick started a collection of banknotes in 1930. He later studied philosophy, literature and history, and by 1964, worked as the manager of a publishing house before dedicating himself to notaphily, the study of paper money, and becoming an acknowledged expert.

When his private collection of (at that time) 180,000 notes became too extensive for a private collector, Pick's collection was received by the Bavarian Mortgages and Exchange Bank (Bayerische Hypotheken- und Wechselbank, now HypoVereinsbank). Pick continued to expand the collection between 1964 and 1985, as a curator in the service of the bank.

Albert lent his name to the Pick-numbers system, whereby collectors can unambiguously identify and catalogue each banknote.

Besides his best known work, the Standard Catalog of World Paper Money,[1] Pick published numerous books abroad and received several international prizes and honors for his publications.