Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 112, § 238

(a) No person shall engage in work as a master, journeyperson or apprentice nor shall any person advertise or represent that he is a master, journeyperson or apprentice unless that person holds a valid license issued by the board as provided in this chapter.

(b) The license or certificate of a master, journeyperson or apprentice shall be displayed when required by an inspector of buildings or an inspector acting on behalf of the board. The inspectors may enforce this section by entering into an establishment where the licensees are performing their work.

(c) A sign, listing or advertisement of a sheet metal business, master or that of a journeyperson sheet metal worker shall contain his designation and license number.