§ 483. Signs advertising services as attorney at law. It shall be unlawful for any person to maintain on real property or to permit or allow any other person to maintain, on such property a sign, in any language, to the effect that an attorney-at-law or legal services are available therein unless the full name of the attorney-at-law or the firm rendering such services is set forth thereon. In any prosecution for violation of the provisions of this section the existence of such a sign on real property shall be presumptive evidence that it was placed or permitted to exist thereon with the knowledge and consent of the person or persons in possession of said premises.