Sarge, I would say we will never know exactly the true weights of the .999 silver inserts. Or at least not until someone is willing to destroy their set by popping out the inserts and weighing them. Then add that to the partial silver mix in the rims.
All this being said, I would just go with the 6.4 with the ? mark. Though it could be a tad more.
My 2 cents,