Maryland Code, Business Regulation § 9A-401

(1) the license and the license number conspicuously in the principal place of business of the licensee; and

(2) the license number of the licensee on each vehicle used on the job for providing heating, ventilation, air-conditioning, or refrigeration services.

(b)(1) Except as provided in paragraph (2) of this subsection, a county or municipal corporation may not require a person licensed under this subtitle to display a county or municipal corporation certificate number on each vehicle used on the job for providing heating, ventilation, air-conditioning, and refrigeration services.

(2) This subsection does not apply to Anne Arundel County.