Looking at the build doc, there is a clipping mod mentioned (see photo) but not a lot of explanation for yours truly. Is the SPDT switch shown of the “on\off\on” verity? Meaning the in the “off” position it would use the diodes on the PCB and then when switched to either of the other two positions, it would use the diodes on the switch PCB.
The schematic indicates that the A and B pads permit the addition of an external DPDT switch which can use one of Barry;s nice mod boards to install other clipping components as shown. When using this mod board, you would not install clipping components on the main board. If you did, then the clippers on the main board would also be in the circuit with the external clippers unless you used a ON/OFF/ON switch. Clear now?