The black valve cover engines were definitely -3 (Special High Performance - more in a bit). There was the 'lil Red which was supposed to be -3 (some were stamped HP) but it had chrome valve covers. Most of the Super Coupes I have seen had the black valve covers, but 2 of them I checked were stamped HP not -3. Then there was was the 300. It had blue valve covers and chrome looking oval sticker on the left valve cover reading "Premium 360". All those engines I've torn down or inspected had all the -3 parts but were stamped HP. All the above were the '69 up 340 cammed engines. Then there was the same stamped HP blocks that had the 252 duration "2bbl" cam option code E57.
My brother had a 79 T-top Magnum 360 2bbl. Ran STRONG for a 2bbl boat. Never purred at idle like my dad's 79 360 Cordoba. Block wasn't stamped -3 or HP but it had a 340 cam in it with a link chain. Engine appeared to have never been apart before. I've spoken with several techs who worked at dealerships during that era and they have told me similar stories of this kind of assembly error both good and bad (high stall converters in 318 2bbl 2.45 4drs, high rpm shift governor in slant six volare, etc.)