Horror House, Brant: The Fear Merchants. V1. 23 Dec 2008

[crutledge]

Secret Agent X was the title of a U.S. pulp magazine published by A. A. Wyn, and the name of the main character featured in the magazine. The magazine ran for 41 issues between February 1934 and March 1939.
The Secret Agent X stories were written by more than one author, but they all appeared under the "house name" of Brant House. The first Secret Agent X story, The Torture Trust was written by Paul Chadwick, who went on to write at least fifteen others. Later stories were produced by G. T. Fleming-Roberts, Emile C. Tepperman and Wayne Rogers.
In the stories, the true identity of Secret Agent X is never revealed. He is a master of disguise, known as "the man of a thousand faces", who adopts several different identities in each story. Although he is a dedicated crime-fighter working undercover for the U.S. government, this is unknown to the police who consider him an outlaw. His true role is known only to newspaper reporter Betty Dale and his mysterious Washington controller, K-9. Agent X come close to being undone once by a woman who could recognise him no matter what his disguise. She turned out to be blind and recognised his manner of walking.
Wikipedia [Thanks, mtravellerh, for the pointer]