The National Industrial Recovery Act, passed in 1933. It included this section:

SEC. 7. (a) Every code of fair competition, agreement, and license approved, prescribed, or issued under this title shall contain the following conditions: (1) That employees shall have the right to organize and bargain collectively through representatives of their own choosing, and shall be free from the interference restraint, or coercion of employers of labor, or their agents, in the designation of such representatives or in self-organization or in other concerted activities for the purpose of collective bargaining or other mutual aid or protection;

This is where the concerted activities protections come from under the National Labor Relations Act (aka the Wagner Act, passed in 1935). I’ve wondered for a while where that language came from and why. I still do. Knowing it came into the NLRA through the NIRA doesn’t answer much at all.