Albert Magnus Miller appeared in the 1927/28 Juror Lists as a port captain working for Admiral Orient Line. He also appeared on the wartime American and British lists of internees at Stanley Camp. Occupation given was master mariner.
In Prisoner of the Japs, Gwen Dew mentions "I remember watching Captain Albert Miller, sixty-seven years old, who began working each day at five a.m. to have sterilized water ready for the Americans when they rose." This gives an approximate indication of his year of birth as being around 1873/74.
As an American, Capt. Miller declined repatriation and remained in Stanley Camp until the end of the war.
Source
1. https://www.chinafamilies.net/internees/9003-miller-albert-magnus/
2. The Shanghai Evening Post and Mercury dated 23 April 1943. See here