The serialization of pistols is similar to that for rifles. The Japanese standard was to add a Kana symbol before the serial number to indicate each 100,000 block of numbers. This kept the numbering to a maximum of 5 digits. So when there is no Kana character (non-Kana series), that is the first block (#1 through 99,999). Collectors refer to this as "no series" or "original series". The Kana series order "IROHA" comes from a Buddhist poem.
All Nagoya Nambu pistols were manufactured at the Kokubunji factory in Nambu's private company, Nambu Rifle Manufacturing Company. Below are the reported serial ranges. Remember that Nagoya arsenal, original series, Chigusa factory, ended production in 7,11 with serial number around 7824.
Nagoya Nambu, original series (no prefix), began in date 8.12 (serial # around 7830) through date 16.10 (serial # 99,999)
Nagoya Nambu, 1st series, date 16.10 (serial # 0) through date 19.8 (serial # around 20,300 when production ended)
Hope this helps.