RCW 47.42.130: Permit identification number.

Every permit issued by the department shall be assigned a separate identification number, and each permittee shall fasten to each sign a weatherproof label, not larger than twenty-eight square inches, that shall be furnished by the department and on which shall be plainly visible the permit number. The permittee shall also place his or her name in a conspicuous position on the front or back of each sign. The failure of a sign to have such a label affixed to it is prima facie evidence that it is not in compliance with the provisions of this chapter.