This is the Futaba SBS-01V Voltage Sensor.

FEATURES: Compatible with the Futaba 18MC-WC, 18MZ, 18SZ, 14SG, 10J, 3PV,
4PX-R, 4PX, 4PV, 4GRS and 4PLS systems
Designed for use with Futaba Telemetry systems, this sensor indicates
voltage of item to which it is attached
Measures two types of voltage such as a main battery used to power a
motor as well as receiver batteries
One year limited warranty throuh Hobby Services beginning at date of
purchase

INCLUDES: Futaba SBS-01V Voltage Sensor with Fuse and Heat Shrink Tube and
Instructions

SPECS: Range: EXT line voltage, 0.0 - 100.0V
Normal line voltage, 3.5 - 8.4V
Total Length: 34.2" (870mm) including wires
Weight: 0.211oz (6.0g)
Voltage: DC3.7 - 7.4V