Section 57A. The provisions of subdivision (3) of section six, the provisions of section eight and the provisions of subdivision (4) of section sixteen, shall apply to any veteran whose retirement is to become effective on or after January first, nineteen hundred and forty-six, under the provisions of section fifty-six or fifty-seven; provided, that the retiring authority shall act in place of the retirement board in the case of any such veteran so retired; and provided, that the expense of any medical examination of any such veteran shall be paid from the same source from which his compensation was paid.