I'm about 95% sure the patent was, as others have said, for the oval seat stays.My impression was that it was for the ovalized seat stays, though I'm surprised that could be patented.
There was a lot of internal talk at the time (not sure about external marketing efforts) about the side-to-side stiffness it provided. I know there were deflection tests that were done as proof the rear wheel flexed less when compare to steel frames and "screwed and glued" aluminum frames.