Nothing in this chapter shall apply to special police officers nor to police officers appointed for temporary duty only, nor to temporary appointees, nor to persons specially appointed as traffic guards, or for the purpose of aiding or directing traffic at or near schools, even though such persons might be endowed with certain police powers, but this chapter shall apply to officers serving initial probationary periods, and such latter officers shall make contributions in the same manner as other police officers. (R.O. 1959, Ch. 32, Sec. 18)