Section 28I. Notwithstanding any other provision of law, an employee of the commonwealth and a member of its retirement system selected to serve with an interstate commission on which the commonwealth is represented and to which the commonwealth contributes, shall, while serving with such interstate commission, continue in membership of the appropriate retirement system, and shall contribute each month to the retirement fund in an amount which he would have contributed had he remained in the service of the commonwealth, and shall be entitled to all the benefits and privileges of the retirement system, as though he continued to be paid from a state pay roll.