K-3 is a new body!
The K-3 has a new body for the first time since the K-7. The K-7, K5, K5II(s) all had Polycarbonate back/side shells. the body is made of four pieces. Top plate, front plate, rear plate and bottom plate. All three were Magnesium alloy metal except the rear plate up until the K-3. The right hand grip is larger. Its a bit, just a bit different body. this is why the battery grips are different.
The K-3 is Pentax first dSLR with a full Magnesium alloy shell, or body. Its not a big deal, but no one has stated it yet in reviews.