Looks like our count is off in the parts list. My apologies. Looks like three possible solutions:
1) You can skip it. Might make a little better electrical contact between the screw and the bell end with the lockwasher in there, which makes it a little better in terms of safety in the phenomenally unlikely event that the power trans shorts to its core. Probably makes good enough contact anyway, this is just belt and suspenders.
2) you can contact Eileen and she will be happy to mail you a washer. queen at bottlehead dot com
3) should be easy to find at Home Depot, Ace Hardware, etc.