TG:
The corner joint you use will depend on whether or not you are a Sobonite or a Bensonite
The drawing you have posted, IMHO, is leaning towards the Bensonite side of timber framing.
I consider this joint a Sobonite joint (although it is drawn with 8x8s and the layout is 2" and 2", the principal is the same with 6x6s, except the layout is 1.5x 1.5"...
Jim Rogers