I'm quite certain there were no washers under the wing nuts. The rubberized coating was thick and soft enough that the wingnuts would bed themselves into the coating. That would prevent them from backing off.
Today I added some water and fuel and as a matter of fact drove the car for the first time since exactly 10 years ago!! Wow, what a feeling. Me and my oldest son loaded up the car for transport to Ettings Maskin & Service where we will adjust the steering and check the emissions.