The CT-P35RLT-18 from Cable Techniques is an 18" cable designed to connect the tape output of Sound Devices production mixers to send a mono mix to a Lectrosonics transmitter.
This 3.5mm TRS right angle to TA5F cable is compatible with the Sound Devices 633 (X3/X4), 664, 688, 552, and MixPre-D.