The Colt Model 1903 was a popular pistol with the Japanese officers as American quality was very good those who elected to private-purchase pistols often times chose the Colt. There were several different makes and models that they could choose from. The baby Nambu, the Colt Pocket Hammerless Automatic, and the 1910/1914 Mauser. All of which had to be chambered in the .32 Cal. ACP.
Many of these Colt 1903's were known to have been carried by Japaneese Officers during WWII, yet surprisingly few came back to the U.S. as war souvenirs. This is presumably due to the availably of Colt Pocket Hammerless Automatics back in the United States. The average American serviceman was probably far less interested in brining home something he could easily obtain at the local hardware store, then the exotic Japanese made Baby Nambu.
This Colt 1903 Pocket Hammerless was sold to the Yokahoma Specie Bank Ltd of Tokyo, Japan. It was shipped to the Kawaguchiya Firearms Company, Ltd of Tokyo, Japan on January 3rd, 1939 in a 5,000 gun shipment.
Kawaguchiya Firearms made Type 97 flare pistols among other things for the Japanese government during the war and certainly would have supply channels to be able to handle these arms.
It's thought that officers in the bank had a hand in the company itself and foresaw the emerging commercial market for such pistols among military officers, hence the order from Colt in such quantity. It is surmised that nearly half of all 32 caliber 1903 pocket pistols in the serial number range 525,000-537,000 were sent to Japan.