This is a Spektrum RC SLR300 3-Channel SLT Receiver Single Protocol.
Specifications:
Type: SLT Single Protocol Surface Receiver
# of Channels: 3
Modulation: FHSS
Band: 2.4 GHz
Length: 33.4mm
Width: 21mm
Height: 12.6mm
Weight: 6g
Voltage Range: 3.5–9.6V
Antenna Length: 140mm