Section 6. Licenses and certificates issued by the examiners shall be valid throughout the commonwealth, but shall not be assignable or transferable. The examiners shall forward to the board of health of each town, or to the inspector of buildings having control of the enforcement of regulations relative to plumbing and gas fitting in such town, the names and addresses of all persons in such town to whom such licenses have been granted. Licenses shall be issued for two years and may be renewed every two years on or before May first, or in case of absence, sickness or other disability of the holder, on or before such later date as the examiners may permit, upon payment of the required fee. Each holder of a master plumber’s or a master gas fitter’s certificate or license shall register his name and business address with said inspector of buildings if he has such control, otherwise with the board of health, in the town wherein he desires to engage in business as a master plumber or a master gas fitter. The fee for such a registration shall be set by the board of health of each town, but in no event shall any such fee be greater than twenty dollars. Any such license or certificate may, after notice and hearing, be suspended or revoked by the examiners upon the violation by the holder thereof of any statute, ordinance, by-law, rule or regulation relative to plumbing or gas fitting, upon failure or refusal of the holder thereof to comply with the rules and requirements of the examiners, or for other sufficient cause. In case of failure to renew a license aforesaid on or before May first in any year or such later date as the examiners may permit as aforesaid, the person named therein may, upon payment of the said fee and, at the discretion of the examiners, a deferred renewal fee to be determined annually by the commissioner of administration under the provision of section three B of chapter seven, increased by such additional fees as would have been payable had such license been continuously renewed, receive a deferred renewal thereof which shall expire on the ensuing first day of May; provided, that such renewed license shall not constitute its holder a licensee for any period preceding its issue.