Section 58B. A veteran who is entitled to be retired under the provisions of section fifty-eight may, on or before the date of his written application for retirement, elect to receive a lesser yearly amount of pension payable to such veteran during his lifetime, with the provisions that upon his death leaving as a survivor his spouse or other eligible beneficiary, as hereinafter defined, at the time of his retirement two-thirds of the yearly amount of such lesser pension shall be continued during the lifetime of and paid to such spouse or other eligible beneficiary; provided, however, that the surviving spouse or other eligible beneficiary shall receive not less than two-thirds of the pension such veteran is receiving at the time of his death; and provided, further, that if such surviving spouse or other eligible beneficiary dies on or after the date such lesser pension becomes effective and before the death of such veteran, such veteran thereafter shall be paid a full pension and may not choose another option. Such full pension shall be determined by multiplying the amount of the lesser pension at the time of the death of such surviving spouse or other eligible beneficiary by a fraction the numerator of which is the yearly amount of the full pension which such veteran would have received under section fifty-eight at the time his retirement allowance became effective if he had not elected that it be paid in accordance with the terms of the first sentence of this section, and the denominator of which is the yearly amount of the lesser pension which such veteran received at the time his pension first became effective.

The election of this option shall be filed with the retiring authority in writing on a prescribed form on or before the date of the written application for the retirement of such veteran, and if the spouse or other eligible beneficiary dies before the date such retirement becomes effective this option shall not take effect. The yearly amount of such lesser pension shall be determined so that the value, on the date such pension becomes effective, of the prospective payments to such veteran, including those for a full pension made in accordance with the first paragraph of this section, and to such spouse or other eligible beneficiary shall be the actuarial equivalent of the value on such date of the full pension that such veteran would be entitled to under the provisions of section fifty-eight.

The computation of the actuarial equivalent of the pension payable to a veteran and to his spouse or other eligible beneficiary under the provisions of this section shall be subject to the supervision and verification by the actuary appointed by the public employee retirement administration commission in accordance with the provisions of section twenty-one and the expenses for such service shall be paid by the governmental unit granting the pension.

If a veteran entitled to be retired under the provisions of section fifty-eight dies before making written application for such retirement, or, having exercised the option provided by this section, dies before the effective date of his retirement, his widow shall receive an annual allowance consisting of two-thirds of the actuarial equivalent to which said veteran would have been entitled had his retirement allowance been computed under the provisions of this section as of the date of death of said veteran, and payable from the same source; provided that said spouse and the deceased veteran were living together at the time of his death, or that the retiring authority finds that they had been living apart for justifiable cause other than desertion or moral turpitude on the part of the spouse.

Any allowance provided for under this section shall be in the alternative to any allowance provided for under section twelve. If the deceased veteran was a member of a system established under sections one to twenty-eight, inclusive, the provisions of paragraph (c) of subdivision (2) of section eleven and Option (d) of subdivision (2) of section twelve shall apply unless the appropriate retiring authority, as defined in section fifty-nine, is notified in writing of the election of the pension under this section within ninety days from the date that the board mailed notice regarding the right of such election; provided, that no pension shall be paid under this section if the deceased veteran is survived by a beneficiary appointed under Option (d) of subdivision (2) of section twelve other than his spouse.

An eligible beneficiary, other than the veteran's spouse, shall, for the purposes of this section, be a child or children under age eighteen, father, mother, sister or brother of such veteran. If a spouse receiving an allowance as beneficiary under this section dies leaving any children of such veteran and of such spouse who are under age eighteen, such amount as would have been paid to such spouse shall be divided into such number of equal shares as there are such children, and each such share shall be paid to a guardian for the benefit of each such child until the child reaches age eighteen.