Section 750.710 - Landmark signs(a)23 U.S.C. 131(c) permits the existence of signs lawfully in existence on October 22, 1965, determined by the State, subject to the approval of the Secretary, to be landmark signs, including signs on farm structures or natural surfaces, of historic or artistic significance, the preservation of which is consistent with the purpose of 23 U.S.C. 131 .(b) States electing to permit landmark signs under 23 U.S.C. 131(c) shall submit a one-time list to the Federal Highway Administration for approval. The list should identify each sign as being in the original 1966 inventory. In the event a sign was omitted in the 1966 inventory, the State may submit other evidence to support a determination that the sign was in existence on October 22, 1965.(c) Reasonable maintenance, repair, and restoration of a landmark sign is permitted. Substantial change in size, lighting, or message content will terminate its exempt status.