3/8" Stainless Steel Engine Bolt Washers, but with Reduced Outer Diameter, which are correct for some Norton 3/8" engine bolts as used on SOHC engines: Price is for bag of 10. Note though, with 3/8" some bolts fitted to Norton chassis, a full width washer can be better to spread load, I normally keep a selection of both.
Trying to find the correct 'reduced outer diamater' type washers that were fitted to Norton Engine bolts is really difficult, particularly in stainless steel. These washers are made specially for this purpose - hence not as cheap as standard washers. Perfect for your Norton engine build though