285 / | HD | PIPE JOINTS OR COUPLINGS |
120.1 | DF | HAVING PLURAL INDEPENDENT PATHS {4} |
123.1 | DF | .~ For concentric tubes or pipes (e.g., well casing or wellhead type) {4} |
123.15 | DF | .~.~ Double-walled or duplex joint {1} |
123.16 | .~.~.~ Wall configured as mating complementary type (e.g., hot air ducts) |