Known as J. Arthur Duff or Arthur Duff. His papers and documents reside at the Hoover Institution Library & Archives at Stanford University: https://oac.cdlib.org/findaid/ark:/13030/kt258032d8/entire_text/
Duff was born in China in 1899 to Canadian missionary parents. Later he was sent to live with relatives in Ontario, Canada. Returned to China in 1920 and went into business in Shanghai. In 1926, he married Jeannie Woodbridge, an American citizen who had a long Christian family background. Duff was fluent in Mandarin and Cantonese.
He was on a business trip in Hong Kong at the time of the Japanese invasion. He managed to hide and elude capture and on 9 January 1942 successfully escaped to Free China via Bias Bay. Duff was recruited by the O.S.S. for the duration of the war. After the war, he had a successful manufacturing and retail business in the Far East. He retired in 1960 and passed away in 1996, and is buried in Palo Alto, California. See https://www.findagrave.com/memorial/76419919/james-arthur-duff
Duff wrote a book "Escape from Hong Kong to Free China, January and February 1942" that was printed by the SCMP. Not dated, circa 1958.
Further reading:
1. His time in the O.S.S. is mentioned in "Asian American Spies" by Brian Masaru Hayashi here
2. The book "Escape from Hong Kong, Admiral Chan Chak's Christmas Day Dash, 1941" by Tim Luard mentions Duff being an American banker "who had slipped out of occupied Hong Kong on 8 January".