Consent, etc., of a competent authority may be proved by writing under his signature

Consent, etc., of a competent authority may be proved by writing under his signature. Whenever under this Act, the doing or the omitting to do anything or the validity of anything depends upon the consent, approval, declaration, opinion or satisfaction of a competent authority a written document signed by a competent authority purporting to convey or set forth such consent, approval, declaration, opinion or satisfaction shall be sufficient evidence thereof.