Having recently purchased "hardware" store washers for some Martano rims, here is what I found.
The original Martano washers are a non standard size, and in the day were raw steel.
A metric washer designed for a 4mm fastener worked best, do note depending on the supplier the plated versions or the stainless versions I found to have a slightly different outside diameters. I stayed away from #8 U.S. Domestic sized washers as they in section would let the rim carry primary contact and often it seems are cut from uneven stock. SAE washers have better tolerance but not happy diameters.
I did wish they were a tad thicker but the 4mm got the nod, in "cad" plate (slightly larger outside diameter), primed prior to use. The primer will I am sure be worn off the edge by the spoke, just wanted a slight barrier between the surfaces of the washer face and rim.
Nisi, way back had some interesting washers that were formed liike a finish washer designed to accept a countersunk screw, which describes a spoke nipple pretty well. They were filled with material that long ago I was at a loss to identify that appeared to help distribute the load and or provide a seal of sorts.
John Jorgensen
back to work in
Torrance Ca USA