The governing body of any locality is authorized in its discretion to make allowances by appropriation of funds, payable in monthly or semimonthly installments, for the relief of any of its officials, employees, police officers, firefighters, sheriffs or deputy sheriffs, town sergeants and town deputy sergeants, or their dependents, who suffer injury or death as defined in Title 65.2, whether such injury was suffered or death occurs before or after June 29, 1948 (which date is the effective date of the section). The allowance shall not exceed the salary or wage being paid such official, employee, police officer, firefighter, sheriff or deputy sheriff, town sergeants and town deputy sergeants, at the time of such injury or death, and the payment of the allowance shall not extend beyond the period of disability resulting from such injury. In case death results from the injury, the allowance may be made for the dependents as defined in Title 65.2. In localities which have established retirement or pension systems for injured, retired or superannuated officials, employees, members of police or fire departments, sheriffs, deputy sheriffs, town sergeants and deputy sergeants, or for the dependents of those killed in line of duty, the agencies provided for the administration of such systems shall determine the existence of such injury or cause of death before any appropriation to pay such allowance is made and shall determine the extent of and period of disability resulting from such injury and the cause in case of death. All sums paid to any such official, employee, police officer, firefighter, sheriff or deputy sheriff, town sergeants and deputy sergeants, as compensation under Title 65.2 and all sums paid to the dependents of such official, employee, police officer, firefighter, sheriff or deputy sheriff, town sergeant and deputy sergeant, if he is killed, and all sums paid under any retirement or pension system shall be deducted from the allowance made under this section in such installments as the agency determines. If the agency determines that any official, employee, police officer, firefighter, sheriff or deputy sheriff, town sergeant and deputy sergeant, who suffered injury in the line of duty is engaged or is able to engage in a gainful occupation, then the allowance shall be reduced by the agency to an amount which, together with the amount earnable by him, equals the allowance. Should the earning capacity of the official, employee, police officer, firefighter, sheriff or deputy sheriff, town sergeant and deputy sergeant, be later changed, such allowance may be further modified, up or down, provided the new allowance shall not exceed the amount of the allowance originally made nor an amount which, when added to the amount earnable by him, exceeds such allowance. The death of, or any condition or impairment of health of, any member of a local police department, or of a sheriff or deputy sheriff, caused by hypertension or heart disease resulting in total or partial disability shall be presumed to have been suffered in the line of duty unless the contrary be shown by competent evidence; provided that prior to making any claim based upon such presumption for retirement, sickness or other benefits on account of such death or total or partial disability, such member, sheriff, or deputy sheriff, shall have been found free from hypertension or heart disease, as the case may be, by a physical examination which shall include such appropriate laboratory and other diagnostic studies as such governing body shall prescribe and which shall have been conducted by physicians whose qualifications shall have been prescribed by such governing body. In the case of a claim for disability, that any such member, sheriff, or deputy sheriff, shall, if requested by such governing body or its authorized representative, submit himself to physical examination by any physician designated by such governing body, such examination to include such tests or studies as may reasonably be prescribed by the physician so designated. Such member, sheriff or deputy sheriff, or claimant shall have the right to have present at such examination, at his own expense, any qualified physician he may designate. In the case of a claim for death benefits, any person entitled to make a claim for such benefits, claiming that such person’s death was suffered in the line of duty, shall submit the body of the deceased to a postmortem examination to be performed by the medical examiner for the county, city or town appointed under § 32.1-282.