If you've lost the little selector off the top of your Illumina, you probably need this little washer. It's what holds the adjuster in place with a little bit of friction.
Please note that the shipping cost will be a significant part of the price of this part. That's the nature of small, inexpensive light parts unfortunately. The best way to avoid that is to package this part in a shipment with others.
Fits both front and rear Illumina for the NA and the fronts only for the NB.