Here is the whole skinny on the rust treatment for the 911 series cars:
Body Galvanizing in Porsche 911
Thyssen steel = steel with Zinc layers on both sides; thickness of the Zinc layers varied from 10 um to 20 um, depending on exposure conditions (Frere, p. 201)
1970 – Galvanized steel (not Thyssen type) used in floor pan and wheel arches (Bob White in Pano V: 142); the “entire platform, including the floor, the longitudinal members, the wheel arches and the seat pans were galvanized.” (Boschen & Barth, p. 124)
1971 models – had galvanized steel in areas particularly exposed to rust (Frere, p. 201)
1972-1973 – Thyssen steel use began (Bob White in Pano V: 142)
1973 – Thyssen steel: rocker panels, inner rocker panels, floor pans, & some other parts (Chuck Stoddard in Pano V: 142)
1975 – Thyssen steel: entire body treated (Bob White & Chuck Stoddard {began in Feb. ’75 production for US cars; earlier for Euro cars – had trouble with paint} in Pano V: 142, 192)
1976 models – had Thyssen steel in entire body except roof (Frere, p. 201)
1977 models – on – had Thyssen steel in entire body (Frere, p. 201); an “important step forward was the use of zinc-coated sheet steel for the manufacture of the entire body structure (except for the coupe’s roof on early production cars). Coated on both sides, this made the structure virtually immune against the formation of rust….” (Boschen & Barth, p. 143)
The Zinc steel added 22 lbs. to the weight of the body structure (Frere, p. 202)
Pano = Panorama volume: page number
Frere = Frere, Paul. 1997. PORSCHE 911 STORY. 6th ed., Patrick Stephens Ltd. Newbury Park, CA.
Boschen & Barth = Boschen, Lothar and Jurgen Barth. 1978. THE PORSCHE BOOK: A DEFINITIVE ILLUSTRATED HISTORY. Argo Books.
Note: the Panorama and Frere comments can be harmonized since most cars for one model year are constructed in the previous chronological year
AJK