I think I just found an answer to my own question. A vintage technical reference that I have mentions that reaction forces with the water naturally create a thrust load on the impeller, presumably in the direction away from the cover. In this pump, that load appears to be taken up by the face of the shaft bushing. I believe that washer called out in the parts catalog is present in my pump. I had assumed that what I was seeing was just a shoulder on the shaft.
Anyway, can someone confirm that I’ve understood this correctly?