Magna is the brand name of an acrylic resin paint, developed by Leonard Bocour and sold by Bocour Artist Colors, Inc. in 1947. It is very different from modern acrylic paint, as it is composed of pigments ground in an acrylic resin brought into emulsion through the use of solvents. Magna paint has more of a shine to it than paints now- a glossier finish. Bocour Artist Colors developed a "true" acrylic paint (development of magna) in 1960 named Aqua-Tec. Modern acrylic paint is water-soluble, while Magna is miscible with turpentine or mineral spirits, though both can dry rapidly to a matte or glossy finish. It was used by artists such as Barnett Newman, Morris Louis, and Roy Lichtenstein. Roy Lichtenstein used magna with oil paints.
Magna (paint) Wikipedia
Golden Artist Colors produces Mineral Spirit Acrylics  as a direct replacement for Magna. Any differences between the two are small, if any, and Roy Lichtenstein used them for that purpose. If anything, he thought the available color spectrum of MSA Conservation Color was better than Bocour's Magna.