Here is how to get a 120, just swapping one pair at each change. Like "Sixty on Thirds" the treble plain hunts throughout, with a different pair swapping each time the treble leads or lies behind. The other four bells ring an extent on four bells, in this case Double Canterbury Minimus (although the text also refers to the possibility of using Double Court Minimus instead). The half-hunt bell (here the 2nd) referred to in the Doubles corresponds to the hunt bell in the Minimus.
The text the goes on to describe the various combinations of whole hunt and half hunt bells that can be used.