from the horse's mouth (EPA/CARB at SEMA)

Back in the good old days, 1961-63, I worked at a major oil company owned service station. They regularly drove in and graded-ed us on what we found or didn't find and what grade of gasoline we we asked to put in and so forth.
It was really something what all we looked at/checked while filling those large tanks back then after finding where the filler necks were hidden.