This is the Futaba R603FF 2.4GHz FASST 3-Channel Receiver.  Used in the 3PM Pistol Grip Radio.

 

FEATURES: Dual Antenna Diversity allows 2.4GHz FASST Futaba transmitter to select the best reception between the two receiver antennas with no signal loss.

SPECS: Reception Antenna: Diversity type (internal and external).  Power requirement: 6V battery.  Size: 1.5 x 1 x 0.55" (39 x 26 x 14mm).  Weight 0.50oz (14.1g)