In any city or town which accepts the provisions of this section, a member of the police department assigned to photographic or fingerprint identification work in said department shall, after completion of one year's service in such assignment, receive in addition to his annual salary as a police officer a salary increase of six hundred dollars a year, to be paid him so long as he is assigned to said photographic or fingerprint identification work.
Mass. Gen. Laws ch. 41, § 108I