Section 85H. The selectmen of every town may retire from active service any call fire fighter or reserve, special or intermittent police officer who becomes permanently disabled mentally or physically by injuries sustained through no fault of his own in the actual performance of duty as such fire fighter or policeman. A person so retired shall receive an annual pension equal to two thirds of the annual rate of compensation payable to a regular or permanent member of the police or fire force, as the case may be, thereof for the first year of service therein, and if there are no permanent members of such force an annual pension in the sum of three thousand dollars. Whenever a call fire fighter or any member of a volunteer fire company in a town whose service as such has been approved by the board of selectmen of such town or reserve or special or intermittent police officer of a town, or a reserve police officer or reserve or call fire fighter of a city is disabled because of injury or incapacity sustained in the performance of his duty without fault of his own, and is thereby unable to perform the usual duties of his regular occupation at the time such injury or incapacity was incurred, he shall receive from the city or town for the period of such injury or incapacity the amount of compensation payable to a permanent member of the police or fire force thereof, as the case may be, for the first year of service therein, or if there are no regular or permanent members of the police or fire force thereof, at the rate of three thousand dollars per annum; provided, that no such compensation shall be payable for any period after such police officer or fire fighter has been retired or pensioned in accordance with law or for any period after a physician designated by the board or officer authorized to appoint police officers or fire fighters in such city or town determines that such incapacity no longer exists. All amounts payable under this section shall be paid at the same time and in the same manner as, and for all purposes shall be deemed to be, the regular compensation of such police officer or fire fighter. No town shall pay compensation under this section if insurance providing coverage for such compensation is in effect therefor under clause (1) of section five of chapter forty, unless and until all rights under such insurance in favor of such town shall have been exercised, determined and satisfied.