Here is a Masterson who was enumerated as living in a fire station house with 9 other men in 1910. Since the last name and first inital is the only information given on the census page isn't it possible that whoever was interviewed for the info or the enumerator themself could have made a mistake in recording the first initial?
1910 San Francisco Assembly District 44, San Francisco, California
J Masterson age 29 born abt 1881 Ireland, parents Ireland, single, unknown immigration date, fireman, lodger at firehouse