It looks like you did not receive an answer to your previous query.
The washers on the pin are almost certainly stainless steel preferably type 316, but if you are unable to get 316, 304 will do until you get a chance to buy the better material.
Regarding size, as no one has answered, make the best guess you can and buy washers in that size plus the next one or two sizes above and below. Washers are not expensive and a few extra in the tool box will not go astray. The washer does not have to be really close fitting so you can safely assume it is a standard size.
The Selden mast seems to be all metric, so try and get metric size washers, but again, the nearest imperial size will also do until you have better information. Not sure about other mast brands. When you get somewhere with more facilities you can change it if necessary. All assuming you still have the pin and a wire or split pin to hold it in place.