53 Pa. Stat. § 15331
The Department of Health shall issue to each master plumber, who reregisters as a master plumber for the year one thousand nine hundred and thirty-six, and to every master plumber, who is granted a license thereafter, a license and a permanent license number, stating that the applicant has successfully passed the examination for a master plumber and is entitled to apply for reregistration annually thereafter; provided he has established a place of business. After this place of business has been approved by the Department of Health, he shall be granted a certificate of registration, and the Department of Health shall furnish one metal plate bearing the words "Registered Master Plumber," Philadelphia, and his permanent license number. If additional metal plates are required, they shall, upon written application, be furnished by the department at a cost to be set by the Department of Health; and provided further, that each and every vehicle used by master plumbers and equipped for trade of plumbing, or used for the regular transportation of tools and equipment used for plumbing, must have attached thereto, on its exterior, in a plainly visible position, the herein described metal plate.
53 P.S. § 15331