There are actually two washers that go on the main shaft. One goes at the top below the two shims and the other goes above all of the shims at the drive collar. Shims come in several different thicknesses and are used to remove any end play. All of these parts, and more, come in a rebuild kit which is now over $150.00 USD.
I used to do a lot of distributor rebuilds but the cost of the rebuild kits and everyone going to electronic ignition kind of made it pointless......