You can usually tell if the master cylinder is for drum or disc by looking at the volumetric size of the unit. Disc require more fluid to operate so the size of the resivoir will be larger. Also, the piston is larger on the disc type M/C. The piston size of 1-1/8" or 1-1/4" is about normal for a disc setup when drums pistons are usually 1" or less in order to build higher pressures that drums need. You also need to to do the length of pedal calculations so you don't over pressure your disc and put your 'sweet thing" through the windshield. Good luck.
