I have a 1974 BNI rulebook.
It's about 1/8th of an inch thick.
There was no definition of what constituted an "automobile." However, rule #32 (engines) required "the car's propulsive force transmitted through the wheels" and banned the installation of jet engines, rocket motors, and JATO units.
There were 12 classes for streamliners and 12 classes for lakesters (A, B, C, D, E, F, G, H, I, J, K, X). Streamliners and lakesters didn't have gas, fuel, supercharged, or normally aspirated records.
There were no classes for electric and turbine cars.
There were gas and fuel records for roadsters. There were no gas, fuel, supercharged, or normally aspirated records for modified roadsters and competition couples.
There were six fuel records (A, B, C, D, E, F) and six gas records (A, B, C, D, E, F) for open bikes.
Streamliner motorcycles were required to have a rollcage but there were no rules governing construction of the rollcage.