mason, n.² — Green’s Dictionary of Slang

mason n.²

also mason line

[proper name Mason-Dixon line, dividing the US north and south along the 40th parallel]

(US black) a town’s main street, esp. when it delineates the line between the black and white communities.

1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.
1947		M.H. Boulware Jive and Sl.