Section 95. For the purpose of promoting the public good and in consideration of long and meritorious service of any official or employee who is not entitled to a retirement allowance or pension under the provisions of any general or special law, a city or town may grant an annuity to such official or employee or to his surviving spouse so long as such spouse survives and does not remarry, or, if no surviving spouse to a legal guardian for the benefit of any surviving child who is unmarried and under age eighteen, in such amount as it may determine, but not to exceed one half of the regular annual compensation received by such official or employee or two thousand dollars, whichever is less; provided, that such official or employee has been permanently employed on full-time basis by such city or town for not less than fifteen years; and provided, further, that if any annuity granted hereunder to such spouse or for the benefit of any such child is less than twelve hundred dollars, it may be increased to an amount not exceeding twelve hundred dollars. The annuity provided to be paid to a surviving spouse under this section shall be paid only to such surviving spouse as was married to such official or employee at the time of his retirement.