This circuit uses the TX-2B RX-2B chipset discussed on the previous page. The chip has 5 channels and the circuit uses 4.
Click HERE for TX-2B RX-2B chipset datasheet .pdf


                                             4-Channel Transmitter PC Board

                                                       TX-2B circuit on datasheet


The receiver using the RX-2 chip:


                                                     4-Channel Receiver PC Board


                                                  RX-2B circuit on datasheet

Source: http://talkingelectronics.com