Green’s Dictionary of Slang

(US Und.) a six-month jail sentence.

1926		Maines & Grant Wise-crack Dict.
1931		Irwin Amer. Tramp and Und. Sl. 126: Magazine. – A six months jail sentence. The time one of the unlettered would require to read a magazine.
1946		Charleston (WV) Daily Mail 2 Sept. 6/5: Me schmaltzy gabber couldn’t red a Chicago piano rap, so the D.A. conks me with a whole mag.
1989		J. Morton Lowspeak 96: Magazine – (UK) a six-month prison sentence [...] (US) a 60-day sentence.