[wingnut60]

I believe Grade 8 bolts are marked with the 6 dashes, as seen in Cummins' pics. This is the bolt used on my B&W hitch also.
I like to err on the side of overkill when working with a lot of weight, so use the G8 bolts whenever I think I need them.
However, I also would think both the strength of the bolts and the contact area between the pin box and trailer box would come into play when trying to determine the capacity of the connection between the pin and trailer boxes, plus several other minor items such as the tensile/crush strength of the metal surfaces.
We need an engineer to analyze this for us...
Joe