Those glossy finishes of the late 60s and 70s were the result of excitement over urethanes in paints. Plastic was "forever" and shiny was good! But, centuries of prior shutters had settled on that semi-gloss and flat finish resulting from combinations of oils and readily available pigments like lamp-black and charcoal for black and various oxides from iron for reddish orange or titanium dioxide for white. Hardware, or the other hand, was "blackened" by heating the hardware to cherry-red then "quenching" it in some of that oil. The result was oil penetrating the porous iron and burning to a black finish.