There are two pieces to a rudder keeper. The front keeper provides a friction-free bearing for the rudder shaft and the second, or rear keeper, slides in behind the first to hold everything together.
Most rudders have a large washer under the head.
The forward keeper is cut away at the top so it can be inserted, then slid forward under the washer. Then the second keeper slides in behind it. The fit is neither tight nor sloppy - just easy. The plan shows one wide keeper and one thin one. The only reason to have one thinner than the other is to identify one from the other quickly when inserting them.