I know the term was used very extensively with the Ford Flathead, but it probably started earlier. A flattie 3/4 race was basically a street cam with aluminum heads and probably a dual carb intake, full race a street cam with a fair number of internal mods and a set of higher compression heads and 3 carbs. Then you got into track, super track and on up the scale. Equate them with the Edelbrock line - Performer= 3/4, Performer RPM=full, Torker.........you get the point. Not used much any longer beyond the OLDER true flathead folks if even there