That advertisement appeared in The Japan Advertiser of Tokyo, 20 October, 1921. As far as I can tell, the Iwai Agency existed in Tokyo since 1886, and I've found ads for it in various European papers of the 1910s and 1920s.
Which, to me, is extremely interesting. A private detective agency operating in Tokyo beginning in 1886? A number of Japanese mystery writers created historical Japanese private detectives--two of the more notable were Okamoto Kido's Hanshichi and Kodo Nomura's Zenigata Heiji--but an actual, historical private detective agency in Tokyo, during the Meiji period? Private detectives walking the streets next to samurai? Japanese private detectives active into the 1920s, in fascist Japan? Something as inherently individualistic as a private detective, or something only slightly less individualistic as a private detective agency, operating in a communal society like Meiji Japan?
You all see the potential here, don't you?
(Of course, the "business connections in all the principal cities of the world" quite likely means that the Iwai Agency was just another unofficial branch of the Japanese intelligence service, who were, recall, extremely active in the United States, Europe, China and Russia from the turn of the 20th century. But "hardboiled samurai private eye" is a lot cooler than "spy posing as a private detective.")