There are two different kits for the different 1275/1500 master cylinders. One supposed way to identify which is to look to see if there are markings on the cylinder or not. This is supposed to be for the "marked" ones. It's probably best to take it apart and see if you have an extra "O" ring or not. This is the one with it. Here is the kit without it.

Clutch MC 1275 Kit with 'O' ring

SKU: CMK-101