Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 129, § 46

Each vehicle used for the transportation of equine animals by any person engaged in the business of dealing with such animals, when operated on any way, shall bear a metal license plate, attached to the side of the vehicle to the left of the person operating or driving the same, such plate to be furnished by the director on the payment of a fee of five dollars. Said plate shall be valid for the term for which the license is granted, but shall be returned to the director on revocation of said license. The use of multiple deck vehicles or the so-called “possum belly” vehicle used in the transportation of equine animals is prohibited.