Not sure about 18.5 x 34mm washer ! The OEM size is ID 14.30mm, OD 25.90 and 2mm thick
Be careful of honda sizes listed
The washer for the swing arm is available 90526 268 000
OK, just seen the swingarm bit crossed out.
That was original a chrome washer. Can post you a S/S one if you require Sye