The Listen Only 2.5mm Surveillance Piece is designed to be used along with speaker mics that have a 2.5mm accessory port. The 2.5 mm ear piece attaches one end to your speaker mic and the other end has a clear tube that fits over the outside of your ear and then sits in the ear canal. The Surveillance piece provides you with discrete communications capability so that only you can hear the transmissions coming from your radio.
The Listen only earpiece goes by several different names including:
Listen Only 2.5mm Surveillance Piece
HYS 2.5mm Receiver/Listen
T20 Series Surveillance Listen Only Earpiece
V1-10432 Listen only Ear Piece
KEP-2 Listen Only Earpiece