The plastic disc under the spring is a round part about 5/16" in diameter and 3/16" thick which rides against the distributor shaft, backed up by the spring which keeps the shaft pushed against one side of the hole in the distributor body. Its purpose is to keep the shaft from wobbling and changing the point gap as the housing wears. If the disc is lost, or wears out, the spring rides directly on the shaft and wears a groove in it. The proper positioning of the parts is disc/spring/grease cup/cap. A piece of 5/16" diameter round phenolic plastic long enough to make a couple of dozen wear discs can be bought on Ebay for less than $5.00. That's a lifetime supply for several distributors.
Jerry
