Hey Denny....go to
www.RockAuto.com, click start, Chevrolet, 1975, C10, brakes, scroll down to master cylinder, scroll down to the manual and you have a description and picture of the master cylinders. Suggest you check out the links I posted above to answer your questions about "brakes" of all different kinds. The big difference in master cylinders is the mounting flange hole spread (GM-FoMoCo), the bore size, and outlet configuration, manual/power, drum/drum,disc/drum, or disc/disc. I've found that if you match your application with weight of your vehicle, manual or power and type of brakes, you're going to be in the ball park. Take this info and head for your local parts house.
One of the above mentioned links even tells you specifically which GM master they are selling you by year and application. I don't see any value in ordering when you can get it locally with a lifetime warranty. Hope this helps.
Dave
