If you go to a Bratton's Antique Auto parts hard copy catalog there is a more complete diagram on page 77. It shows item 29A as a steel thrust washer, .015 thick, part number 17390. This washer is placed at the top of the shaft just below the shaft collar. Its purpose is to prevent wear to the cast metal at the top of the shaft tunnel in the casting. Item 4 shows a fiber washer, .048 thick, part number 17400. This washer is placed at the bottom of the shaft just above the distributor shaft sleeve that is pinned to the shaft. Its purpose is to take up play in the shaft. When rebuilding a distributor you may find you need additional washers due to wear to the top or bottom of the casting.
What I do when rebuilding a distributor is place one steel washer at the top of the shaft and one fiber washer at the bottom of the shaft. If there is end play due to casting wear I add one or more steel washers at the bottom just above the distributor shaft sleeve.
Quite often I find that the shaft will bind with just the two washers installed when a new shaft collar is installed. What I do is grind some material off the top of the collar until there is a nice snug fit with no binding and no end play.
Tom Endy