Any county, city or town providing death, retirement, sickness or other benefits pursuant to the authority granted by § 27-39, or pursuant to any other provision of law or the charter of any city or town, or otherwise, shall do so exclusive of, and without regard to, any such benefits paid or payable out of the general fund of the state treasury pursuant to § 9.1-400 et seq. and shall by ordinance make provision for the employment of physicians and the performance of the physical examination required by § 27-40.1 and shall cause such examination to be made within ninety days after June 1, 1973, of every fire fighter in its service or the service of a political subdivision with which it has contracted for fire protection and of every fire fighter entering upon such service thereafter at the time of such entry, provided however, that any fire fighter employed by any such county, city or town which failed to cause such physical examination to be made on or before January 1, 1976, for any fire fighter employed prior to January 1, 1976, in its service or the service of a political subdivision with which it has contracted for fire protection shall be presumed to have been found free from respiratory disease, hypertension or heart disease as if such fire fighter had been examined pursuant to § 27-40.1. Such presumption shall also apply to the benefit of any fire fighter entering upon such service on or after January 1, 1976, unless said county, city or town shall cause such examination to be made of such fire fighter within ninety days after July 1, 1976. Every fire fighter entering upon such service on or after October 1, 1976, and thereafter, shall be entitled to the benefit of such presumption unless such county, city or town shall cause such examination to be made of such fire fighter at the time of such entry.