Whenever the contractor or subcontractor has a collective bargaining agreement or other contract or understanding with a labor union or an agency referring workers or providing or supervising apprenticeship or training for such workers, the compliance report shall include information pertaining to such labor union’s or agency’s practices and policies affecting compliance, as the commission may prescribe; provided, to the extent such information is within the exclusive possession of a labor union or an agency referring workers or providing or supervising apprenticeship or training and such labor union or agency refuses to furnish information to the contractor, the contractor shall so certify to the commission as part of its compliance report and shall set forth what efforts have been made to obtain such information.