Section 9. Any bank book representing a deposit account in a savings bank or trust company within the commonwealth, which belongs to a former patient of said hospital and shall have remained unclaimed for more than two years in the custody of the superintendent thereof, may be presented by the trustees to such bank or trust company accompanied by their written request for payment to them of such deposit account except so much as is in excess of the amount due the commonwealth for the support of such patient, and such bank or trust company shall thereupon pay to the trustees the amount so requested.