The A and B are a way of referring to the High and Low side of the signal. For example, the Tuning out pins would have TxA(Hi) and TxB(Lo). See attached for the description of the pins from Gables NCP pinout.
The H and C stand for Hot and Cold on the power line. The Hot line is the line with the energy, and as such is more dangerous than the Cold line. Boeing recommends refraining from touching either wire while power is on.
The navigation (NAV) control panel supplies frequency inputs and test commands to the DME, ILS, and VOR navigation radios.
737NG SDS 34-51-00.