Every person who was engaged in the trade or business of installing sanitary plumbing on May 4, 1945 shall be entitled to his initial certificate under § 40-49-330 provided application therefor is filed with and the fee provided in said section is paid to the board issuing it within sixty days after the date of the organization of such board. No person holding a certificate granted by a board of plumbing examiners created under § 40-49-20 or under § 40-49-280 shall be required to have a certificate under this article to engage in the business of installing sanitary plumbing or to install sanitary plumbing in any part or section of the county or in any municipality, incorporated community or water, sewer or public service district in the county.