What era is the original reference? Is it USA or Canada, do you know the ethnicity of the original person using the phrase. English is a constantly changing and diverse family of languages.
I have looked in various slang dictionaries, English and American, a railway dictionary, and the Oxford English Dictionary. All l can come up with is that a magazine is some kind of storage device that automatically offers up the next one like a magazine on a pistol or slide projector. Or it can be a military store usually for ammunition and weapons or more rarely anything used by the military. Mag is short for magneto as found on a car. Mag is an obselete prefix for an operator of fraudulent gambling games. A Mags man is a safe breaker iirc.
