This article does not prohibit:
(a) Any lawful activity for the purpose of engaging in any organizational effort on behalf of any labor union, agent, or member thereof, or of any employee group, or any member thereof, employed or formerly employed in any place of business or manufacturing establishment described in this article, or for the purpose of carrying on the lawful activities of labor unions, or members thereof.
(b) Any lawful activity for the purpose of investigation of the safety of working conditions on posted property by a representative of a labor union or other employee group who has upon his person written evidence of due authorization by his labor union or employee group to make such investigation.