One Piece Design. Instead of using a 2 peice design like the #326 bottle valves #660 valves use a 1 piece bottle nut and nipple that is sealed with the supplied teflon gasket.
Application. #660 style bottle nuts are used by some nitrous companies such as NOS and on some mother bottles. Nitrous Outlet, Nitrous Express, Zex and many other nitrous companies have switched to using the two piece #326 design.