Under the Sherman Act, it is unreasonable ... for a manufacturer to seek to restrict and confine areas or persons within which an article may be trade after the manufacturer has parted with dominion over it ... Once the manufacturer has parted with title and risk, he has parted dominion the product, and his effort thereafter to restrict territory or persons to whom the product may be transferred, whether by explicit agreement or by silent confirmation or understanding with the vendee, is per se a violation of section I of the Sherman Act