(a) Signs or bulletin boards customarily incidental to places of worship, libraries, museums, social clubs or societies; such signs or bulletin boards shall not exceed fifteen square feet in area and shall be located on the premises of such institution;
(b) Any sign advertising a commercial enterprise, including real estate developers or subdividers, in a district zoned residential shall not exceed twelve square feet in area and shall advertise only the names of the owners, trade names, products sold and/or the business or activity conducted on the premises where such sign is located.