semi-gloss DOES have gloss (not full gloss), but something less. Even "flat" has a small degree of gloss (as it's extremely difficult to achieve totally flat surfaces - re reflections).
During the last 60's/early 70's all of the American car makers used some 'non glossy' black accents on their performance cars. in some cases, it was much much flatter than what Chevrolet used. The problem with all of those 'non full gloss' paints is that just rubbing them, washing them, polishing them, etc... made them MORE glossy! Have you ever seen the flat black paints after they've been washed, rubbed, etc for a number of years? They look more glossy as a result of all that.
So, talking about how glossy, or non glossy, 44 yr old 'original' paint is NOW.. is subject to a LOT of error sources. I'm old enough to remember how these early Camaros looked when new, and I owned some then, and had many friends with them as well. I bought a new '70 RR, with a non-glossy black on the hood, and I remember like yesterday how difficult it was when polishing/waxing the car, NOT to get anything on the black portion, as it made it look ugly when you got polish or wax on the edges of the black hood. A lot of people would just polish / wax the entire thing, for a consistent look, even though it totally changed the 'gloss level' as painted in the factory.