Okay back to the OP's question. In the early days you had several different promoters putting on what would be considered Nationals. You had Score/Coyote Racing/Baja Tri Cross (3Wheeling Magazine) and others, mostly located on the West coast. It was not until the AMA/AATVA got involved and started what is now considered National races.
It was up to the promoter to determine the classes. I found a program from the 1983 Off Road USA Odyssey-3 Wheeler Championships at Santa Cruz. It is what was going on at the time in regardes to a National type race.
As you can see there was a "Pro" 125/200/250/Open class. But the main classes thru to the end were the 200 and 250 class both in Amateur and Pro.
You have to remember that back in the 80's it is not like today.... Unless you were a camera freak you really didn't take pictures. Now day's everyone has cameras on their phones.
Here is a picture of Mickey "D" for ya. (He won the Main ahead of Curtis, Marty, and Steve)
Some early Kawasaki Days (Jimmies first season on the Kawasaki)
Also for all you Jackie Meadows fans who thought he only raced Kawasakis. Check out "Big" Jack with Jackies Tricky Dicks Conversion CR Honda.