Explain differences in SWR meter circuits
The circuits look similar but rely on two different measurement principles. One uses a current transformer while the other uses two coupled transmission lines.

Fig 7 uses two parallel transmission lines to pick up a fraction of the power from the main line to the secondary line. Note that this will happen in both direction simultaneously. As long as the secondary line is perfectly impedance matched in the opposite end the power picked up in the opposite direction will be dissipated by the termination and never be reflected back. This means that the voltage at the end of the secondary transmission line is only dependant on the power going in the same direction on the main line. The diode rectifies this voltage and sends it to the meter with is calibrated directly in watt.

Fig 8 measures the line current with a current transformer (not a transmission line) and the line voltage with a capacitive voltage divider. The trim cap is adjusted so that the voltage on the cathode of the diode is exactly equal and in phase with the voltage from the current transformer when everything is perfectly matched. In this case the meter will show zero since the voltage across the diode is zero. The other diode will have the same cathode voltage but the anode voltage from the current measurement will be exactly 180⁰ out of phase and you will get twice the voltage across the diode which results in a meter reading.