I think all 1970's lugs were Stamped. Perhaps TREK was one of the first companies to start using investment-cast lugs circa 1983 or 1984 (I bought an investment-cast-lugs Trek 500 Bicycle in 1985). Stamped lugs are lighter and are more versatile because they make it much simpler to change the frame angles. They are mild steel and meant to be cold-worked and have inform wall thicknesses, unlike investment-cast lugs.. A series of dies is used on a flat steel plate to form the lugs, and it is eventually welded and cleaned up. Today, most lugs are investment-cast. A well-made IC lug needs almost no attention from a framebuilder, whereas a 1970s set of stamped lugs would get a lot of attention to square the edges, thin the lugs at the shorelines, and smooth the mottled surfaces, especially if they were going to be chromed.