Works only on RC-50 and RS-50 models or with the CFL-39 complete flow cell assembly.  Screws into the special black tee as seen in CFL-39 image below and in CLF-40 in the flow cell chamber section.  Clear 360 degree viewing of electrodes for monitoring purposes.  Easy to unscrew with no tools or Teflon tape required!  Comes with one “O” ring.  Electrodes are 3” in length, 1 ¾” “half-moon” in diameter.