73. Where a contributor’s child is entitled to an annual allowance or another amount under this plan, that amount shall be paid, where the child is under age 18, to the person who has legal custody of him and is vested with authority over him and, for the purposes of this section, the contributor’s surviving spouse, except where the child lives separated from him, is presumed to be the person who has custody of that child and who is vested with authority over him.