Didn’t Levi’s also have the double-seam on the outside back in the 60’s and 70’s? I seem to remember that....
Never a double stitch on the outside of the leg but sometimes a double stitch on the inside. This is based on a quick scan of vintage 501s on eBay.
And actually the outside doesn’t have a visible stitch at all. Levis uses a butted seam rather than a folded seam (not sure if these are correct terms) on the outside of the leg that hides the stitching and looks smoother.