I have a 1966 Mustang GT, I want to convert my single Master Cylinder to a dual. I ordered the kit for a DIsc/drum and when I opened the pack with the master Cylinder it had a one inch long round piece in the package. I am not sure what it is for or where it would go.

Here is the piece, it is about 1 inch long. Seems like it would fit in the bore of the Master Cylinder

 

Not a great picture but looks like what you would put on a vacuum port to seal it off, assuming it is hollow, as when you don’t have power brakes.