I tore everything apart to install the mystery washer, only to discover that the washer's ID was too small to fit below the dust shield
. After going back through all of my pictures, I think this is another artifact of the prior owner's dubious work. He had the washer between the upper bracket and the steering stem nut.
In other news, OMG, what an improvement to my steering! It feels like a new bike; the front end is so much lighter and more precise. 5-Stars, highly recommended. Do not put off the maintenance of your steering head bearings!