im not so sure your right.. 1976 was the first year the entire car’s were..
https://rennlist.com/forums/911-forum/567159-what-year-did-they-start-zinc-coating.html
"Galvanized steel was in use since the 1971 models, for body parts particularly exposed to rust, but from 1976 models on the entire body was assembled from steel sheet protected on both faces by a layer of zinc the thickness of which varies from 10-20 Thousands of a millimeter, according to the conditions to which it was to be exposed. Only, and for one year only, the roof was not galvanized because units of that size were not available at the time. Porsche was the first car manufacturer in the world to offer such comprehensive rust protection and it took 10 years for another -- Audi -- to follow." Paul Frere; Porsche 911 Story, pg 201