NM Griswold has an unusually high TC (as does NM Smith and, arguably, NM Cow King) but his mlvl is not high for his location in the game (42). What this means is he can drop you base items from way up high but he probably won't be able to drop you the unique versions of them.
At best he can drop a few exceptional set items, like the Orphan's Call stuff. He can't even drop Griswold's armour because the Griz set has a qlvl of 44.
He can as mentioned drop up to a Vex rune though, and is the first creature who can drop such a high rune. But he is staggeringly unlikely to do so (about 1 in 70,000 kills).