Section 18. There shall be an armory commission composed as follows:—The adjutant general, who shall be chairman, the state quartermaster, who shall be clerk, and the air guard commander, Massachusetts national guard. Neither the adjutant general nor the state quartermaster shall receive any additional compensation on account of their membership in said commission. The said air guard commander shall receive one day’s pay of grade and allowances for each day that he actually sits or performs the duties incumbent upon him as a member of said commission, but in no case more than five hundred dollars in any one year. In case said air guard commander is relieved from duty in the national guard by reason of being in the military service of the United States of America under call or order of the president of said United States, or in case said air guard commander, while not in said military service, is unable to perform his duties as a member of said commission by reason of absence from the commonwealth, the governor as commander-in-chief may appoint some other general officer of the land forces of the commonwealth to act, during the period of such service or absence, as a member of said commission in lieu of said air guard commander.