Section 29A. Whenever, within thirty days after notification to the city or town, the board shall have received written objection to an application for a permit, such permit shall issue only after consideration by the board of such objection, and whenever, within thirty days after notification to the city or town, the board shall have received written notice of intention to appear in opposition to the application, the board shall issue such permit only after a public hearing on due notice to the applicant and the city or town. Any applicant for a permit, or any city or town wherein a permit is to be issued, aggrieved by a decision of the executive director with respect to the issuance or revocation of a license or permit for the erection or maintenance of a billboard, sign or other advertising device, may, within fifteen days thereafter, appeal from such decision to the board, which may, after a public hearing held on due notice, order such decision to be affirmed, modified or annulled. The board may in its discretion order a public hearing with respect to any decision of the executive director within fifteen days after such decision, and may after such hearing order such decision to be affirmed, modified or annulled. The findings of the board on matters heard before it shall be final except with respect to matters of law.