At any time following the end of the fifth year after a sign becomes nonconforming the commissioner of transportation may, by registered or certified mail, revoke the permit and order the removal of such nonconforming sign, which order shall be final on the date of mailing. Until the end of said 5-year period no nonconforming sign may be ordered to be removed unless it violates N.H. Rev. Stat. § 236:74, I(a), (b) or (c). In no event, however, may a nonconforming sign be ordered removed from a federal highway adjacent area if its removal would require just compensation to be paid pursuant to N.H. Rev. Stat. § 236:80 and the federal share of just compensation to be paid under Section 131 of Title 23, United States Code, is not available to make such payment.