two diametrically opposite sections on said sleeve, {HEAD ~ each} of {TAIL ~ said sections} having recess means on said inside surface opening toward said axis for receiving one of the transmitter and the receiver and to permit radiation to pass through the body between the receiver or transmitter; and 
of
two diametrically opposite sections on said sleeve , { HEAD ~ each } of { TAIL ~ said sections } having recess means on said inside surface opening toward said axis for receiving one of the transmitter and the receiver and to permit radiation to pass through the body between the receiver or transmitter ; and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH