Conn. Gen. Stat. § 21-56: Permit numbers.

The Commissioner of Transportation shall provide with each permit issued for the display of advertising, under the provisions of this chapter, a permit number which shall be painted on each structure in legible figures not less than two inches in height and at the following locations on such advertising billboards and signs: Poster boards, on the top moulding at a point nearest the highway or street; signs erected on a single post, on the face of the post under the sign. The provisions of this section shall not apply to advertising signs or displays on or in railroad stations intended for display to the patrons of railroads.