Such presumption, subject to the provisions of § 27-40.1, shall be employed in determining eligibility for death, retirement, sickness and other benefits provided pursuant to the authority granted by §§ 27-39, 27-40, or pursuant to any other provision of law or the charter of any city or town, or otherwise for persons who die or become totally or partially disabled; provided, however, that such presumption shall be used to determine eligibility for death benefits for all fire fighters employed by the City of Portsmouth when death occurs subsequent to July 1, 1972.