This may help you, it may not.
I went to the hardware store with a MTB rim in hand last week looking for washers that would fit over the nipple shank and be small enough o/d to fit inside the drilling in the inner rim section. Long story, but often I wish to reinforce non-eyeletted MTB rims a little, or simply re-use spokes in a hub-rim combo that the spokes are a mm or so too long for.
Anyway, standard plated steel washers, think they are #6, are the correct i/d, but are too large o/d..........but the exact same #6 washers in stainless were much smaller o/d and fit perfectly ;-) I was a happy camper.
Neill Currie
Hillsborough, Nh.