This is an optional Futaba SBS-01V External Voltage Sensor, and is designed for use with Futaba telemetry systems. This sensor measure receiver battery voltage and the voltage of a second battery or power supply. 

Specifications:
External Line Range: 0-~100V
Normal Line Range: 3.5-~8.4V
Weight: 0.2 oz. (6.0 g)
Length: 34.25 in (870 mm)