Master cylinder interchange disc/drum-drum/drum

Hello all,

I'm working on a 74 Duster with manual drum brakes all the way around and need to replace my master cylinder. It does not appear as though there are many options left out there and I want to make sure I get the correct(functional) one. How can I look at mine and determine the bore size? It seems that the common ones out there are Cardone 10-1573M with 1.031 bore and 10-1516M with .937 bore. There are other brands Centric and others that I really can't make any sense out of when it comes to the bore size, can anyone provide some insight?

v/r
Ron