Section 48. An officer, instructor or employee who is retired under section forty-six shall be allowed a pension equal to one half of the salary which he was receiving at his retirement. In computing the salary of such an officer, instructor or employee who receives a non-cash allowance in the form of full or partial boarding or housing, the commissioner of correction shall add to the cash payment for regular services an amount which, in case of full boarding and housing, shall not exceed seven dollars a week and, in case of partial housing and boarding, shall not exceed a fair proportion of such amount. The pension, in the case of an officer, instructor or employee of a correctional institution of the commonwealth shall be paid in monthly instalments by the commonwealth; and in the case of an officer or instructor of a jail or house of correction, the pension shall be paid in monthly instalments by the county.