Mark,
There is no callout for a separate washer. There is supposed to be a sealer material as part of the nut/washer assembly (373556-S100) and the sealer "may" have pulled thru the mounting hole in the body if the hole was off center or too large. As an assembly line fix, a washer was used - note "may". As this was the trunk area, there were standards (Ford or federal ?) to not allow exhaust gasses to enter (leak into) the trunk and the interior in general and this "may" have fixed that leak. I looked at the 64-66 manuals and they have a steel washer and sealer washer called out. On the other hand, previous owner may have not had the correct hardware.
Jim