PRB used to quote 2 different listings for the master cylinder. Supposedly, the SX (discontinued) one was of a larger bore. When I did my conversion, I did change mine, but PBR's listings also state that the standard M/Cyl is appropriate for Disc/Disc setups. My advice would be to fit it, assuming its still in working order. I believe there is a small difference in the Bias-ing between the Disc/Drum, Disc/Disc - but for the most part you wont notice it too much...
however, open to corrections again....