Hy comeaud, the GM part number is 685223 that translates to Napa p/n NMC P8568 $79.65 + $15.56 core, this master has ports 1/2"X 20 t.p.i. and 7/16"X 24 t.p.i.and is a 1 3/4" bore. My old United parts book lists p/n 39457 which translates to Napa p/n NMC P8551 $107.10 + $50.00 core this master has ports 1/2"X20 t.p.i. and 9/16"X 18 t.p.i. and is a 1 3/4" bore. Both these master cylinders have a four bolt flange mount, hope that helps.
