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Cabs weren't made until 1982, so the targa design had nothing to do with this.
Sorry, but the 356 had been made as a cab and when they were developing an open 911 Porsche was concerned that "the ever more safety conscious American legislation might ban conventional open cars altogether. It was with all these considerations in mind that Porsche created a completely new body style . . ." Paul Frere, The Porsche 911 Story.