Note the pip in the above which looks as though it should locate something, but doesn't.
Leaf springs added to the swivelly bit. The axle part added with the longer sections between the springs. Period photos are not that numerous. The two here from the NRM and aren't quite the same as the kit although the instructions do state that it's a Midland Railway example.
The swivelly bit is attached lining the rings up and setting here at a jaunty angle to fit the position in the yard on Edge. The join behind the box sections will probably need to be scored to move the horse attaching bits up to the correct position.