Did mid to late 70`s mopars originally have a washer under the nut that holds the tuff wheel adapter or steering wheel on? I don`t remember them having one but the PO must have put one under the nut for whatever reason. I don`t see one in the parts book either.

I don't know for you specific application but do remember a thin steel washer being present under the nuts on several other mopars of the 60- 70's vintage. The OD was about the same size or slightly larger than the distance across the hex points.

i remember some washers as well.
i don't know this for sure, but could it be to insure the splines on the shaft weren't damaged if the wheel was cranked on too tight ?
i have had some wheels that were really hard to remove because the shaft splines somehow got mashed by the nut, effectively swedging the wheel to the shaft.
but then again, anything could have caused that in the past 50-60 years since the car was new.

This particular car is a 77 plymouth volare. I don`t see a washer listed in the 77 parts book. Eberg has one listed on ebay with a washer but the nut is different from mine. https://www.ebay.com/itm/223715167028?ul_noapp=true

The washer pictured on the ebay ad is pretty much what I remember. It's possible that the nut and washer came together under one part number as indicated in the ad