49 CFR § 178.512 - Standards for steel, aluminum or other metal boxes.
(a) The following are identification codes for steel, aluminum, or other metal boxes:
(1) 4A for a steel box;
(2) 4B for an aluminum box; and
(3) 4N for an other metal box.
(b) Construction requirements for steel, aluminum or other metal boxes are as follows:
(1) The strength of the metal and the construction of the box must be appropriate to the capacity and intended use of the box.
(2) Boxes must be lined with fiberboard or felt packing pieces or must have an inner liner or coating of suitable material in accordance with subpart C of part 173 of this subchapter. If a double seamed metal liner is used, steps must be taken to prevent the ingress of materials, particularly explosives, into the recesses of the seams.
(3) Closures may be of any suitable type, and must remain secure under normal conditions of transport.
(4) Maximum net mass: 400 kg (882 pounds).