Pup out or sub out. Super heavy duty, solid 304 stainless steel collar with four rings which attach by Phillips head screws. These rings are not fixed, they rotate. Perfect for play, because you know, attachment points! Collar closes via hinge mechanism.
The lock is a cross lock (the key kind of looks like a Phillips head screwdriver and the keyhole looks like a plus sign) integrated into the collar. Two keys included.