This is the Futaba MSA-10 Servo Synchronizer.
Ideal for Giant Scale Models that require multiple-servo control.
Not to be used with more than 6 volts.
Couples up to Four Servos and electronically balances them to start and stop simultaneously for strong, smooth operation of even the largest control surfaces; plugs into a single Rx channel.
Allows for adjustment of end point and servo travel for each servo.
Also features reversing, so servos can be mounted in the most efficient configuration possible.
Allows these 4 servos to be powered separately from receiver.