i THINK ITS #26 just floating outthere in the picture, no description. try a oil drain plug washer, might fit.
I went to Motogrid.com and same fish. #26 is a 14mm drain plug washer, qnty 1. #25 is a 12mm washer which would be for the drain plug. Still #26 with qnty of 2 seems to be what is required. Of course you only need the one as the other line is OK.