There are s12 and s12 + 8 Calipers. I think one is for vented, and one for solid discs.
I'm running s12 + 8s in the Z. And thats for vented discs.
Also, just FYI. If you need to rebuild them, buy willwoods. As i went the s12+8 route, and even at trade it was ~$600 to buy and fully rebuild them. $300 more and i could have had brand spanking new, lightweight willwoods. The S12 calipers are cast iron and weigh 236498236kg each.