two diametrically opposite sections on said sleeve, each of said sections having recess means on {TAIL ~ said inside surface} opening toward {HEAD ~ 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 
two diametrically opposite sections on said sleeve , each of said sections having recess means on { TAIL ~ said inside surface } opening toward { HEAD ~ 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 OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD 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