You can find some transcriptions of the 1910 Census here:
http://pilot.familysearch.org/recordsearch/start.html#There is a John Masterson who shows up on the 1920 & 1930 Census in San Francisco with an arrival date of 1904 and an age close to the J. Masterson noted above. He & his parents were born in Ireland. He was a police officer, so I'm not sure if he was the same one as the J. Masterson in 1910. The WWI Draft Registration Card that I found for this John Masterson gives a birth date of 3 Apr. 1882 and, based on the California Death Index, he died in San Francisco on 16 Dec. 1968 and his mother's maiden name was Sheridan. This is probably a total stretch from your guy, but I guess a name change is possible, so just thought I'd mention it in case.