Shane, from my observations they did the cross bolt front/rear caps, then they did the center cross bolts (6 bolt mains), then they casted the last batch which have a coolant return boss on the front drivers side of the block for some new S2 heads that never came to light.
So to answer your question, it is not a 6 bolt block, that was dropped for some reason.
This is the same vintage block I run now.