9 C.F.R. § 2.51
(a) The official tag shall be made of a durable alloy such as brass, bronze, or steel, or of a durable plastic. Aluminum of a sufficient thickness to assure the tag is durable and legible may also be used. The tag shall be one of the following shapes:
(b) Each tag shall have the following information embossed or stamped on so that it is easily readable: