Anyone where I could find the minimum R-value specs from the building codes in force in the 1970's?
There probably wasn't any standard. Connecticut was the first state to adopt a mandatory building code that applied uniformly to every municipality in the state, and that didn't take place until 1970. Most other states didn't get that far for at least another decade, and in most states up at least into the 90s each county, city or town might adopt their own code. There used to be three "model" building codes published, that were available for adoption by government jurisdictions. Texas had the unique distinction of simultaneously having versions of all three in force in various parts of the state. Needless to say, it was a nightmare for architects and engineers who had to figure out which code to use for which project.
Depending on where your house is located, it's entirely possible that even if the city or town (or county) had a building code, it didn't specify any insulation values. Back in the 70s energy conservation wasn't on anybody's radar. We intentionally did NOT use insulation in factory roofs, because we wanted as much process heat as possible to get out in order to reduce air conditioning costs in the summer. Heat was cheap compared to electricity for a/c.
