My CM (CR205XX) actually came with two little brass/bronze washers between the screw and guide rod. They are small, 0.1855" diameter and 0.010" thick.
I have heard that the washer was there on later models to keep the aluminum spring cap (guide plug) from beating itself on the stainless screw. Inspecting the front of the guide plug shows that it does work.
I have been meaning to make a couple of plugs but never seem to have the time. Do you have any for sale?