From and after January 1, 1973, all signs erected or maintained along federal aid primary or interstate highways except signs described in section 3, paragraphs (b) and (c), of said highway advertising control act of 1972 shall have attached thereto in the lower left hand corner the permit number of the permit issued for such sign. Such permit number shall be in block letters no smaller than three inches. Said numbers may be painted on or attached in any manner that is durable.
Kan. Admin. Regs. § 36-17-3