The provisions of this chapter governing contributions, pensions, and benefits shall have exclusive application (1) to firefighters as defined in this chapter hereafter becoming members of a fire department, (2) to firefighters as defined in this chapter heretofore employed in a department who have not otherwise elected as provided for in RCW
41.18.160, and (3) to firefighters on disability retirement under chapter
41.16 RCW, at the effective date of this chapter, who thereafter shall have been returned to active duty by the retirement board, and who have not otherwise elected as provided for in RCW
41.18.160 within sixty days after return to active duty.