help 1964 Slant 6 tune ups specs

I completed the install of the NGK plugs, echlin rotor, and Standard Dist cap today. I forgot how hard it is to get the cap clip in, on the left side next to the oil filter, I used a med size screwdriver to press it in place which made the job easy. My car has a simple electronic distributor from a previous owner. I do not have a lot of knowledge about distributors, but it is connected to a blaster coil, with a firewall mounted ballast. My question is: What is the 1/2 inch plastic round washer that sits on top of the distributor shaft, just below the rotor? I am guessing som kind of pick up?