A ball lock version of the William HMS. The Ball Lock works in a similar line to Twist Lock Carabiners that you may be more familiar with. Basically to open the gate you squeeze the green ball, then twist the gate, and then open, after a few practice goes it is very easy to operate.
The William is a large HMS locking carabiner, great for setting up belays as multiple clove hitches are accepted, and therefore easy to maintain a cluster free belay. The fact that it is key lock is a great bonus as the is no chance of getting the nose snagged when setting up. With the rope friendly pear shape, this also makes an ideal belay carabiner.