The washer you mention may be the somewhat large and thin one that helps prevent the spring from being pushed into the filter if it gets clogged (I think the spring works with the bypass). It is listed as"WASHER,20.5X30X1.0 92022-1012". It can get pushed thru the filter's grommet (seal). Its pictured just above the spring. The o-ring in question rolls onto the filter bolt/banjo and seals the filter plate. I usually get quite a few uses before replacing it. On a Honda (GL500) of mine that used the same configuration, the spring would be always pushed halfway into the filter every oil change until I realized it was missing the washer. It seems to not do this on the Concours.
The filters we buy, although specific for our bike, fit many other bikes too. The washer in question might be for a different purpose on those and not required on ours.