For the purpose of interstate and combined interstate and intrastate reciprocity provisions of §§ 56-3-400 to 56-3-420, automobile utility trailers shall be extended full reciprocity, provided that every person who owns automobile utility trailers within this State and who is engaged in the business of leasing such trailers for use in interstate, intrastate or combined interstate and intrastate commerce (a) registers and licenses all automobile utility trailers owned by him in the state wherein the owner actually resides and such state affords equal recognition, either in fact or in law, to such trailers licensed in this State or (b) registers and licenses in this State the average number of automobile utility trailers operated in and through this State during the licensing year. In such instances, the person shall register with the Department of Motor Vehicles the fact that he is engaged in such business and shall file data in such form and verified in such manner as shall be required by the Department, estimating the average number of automobile utility trailers he leases for operation in and through the State during the licensing year. The Department shall then determine the average number of such trailers operated in and through this State during the licensing year and such determination shall be final. Upon payment of the fee required by § 56-3-130, the Department shall, subject to the provisions of § 56-3-420, issue registration certificates and license plates for the average number of such automobile utility trailers as above determined and that number of trailers shall display the license plates. Thereafter, all trailers properly identified and licensed in any state, territory, province, country or the District of Columbia and belonging to such person shall be permitted to operate in this State on an interstate, intrastate or combined interstate and intrastate basis; provided, that such trailers are towed by private passenger cars duly registered and licensed in this State or in another state and legally operated in this State under the reciprocity laws of this State.