levelling camera while in vertical position
This looks like a thick hockey puck and goes between the tripod and the panorama head (I use a Nodal Ninja 3).
The thickness is needed to house a lockable ball that makes it easy to establish level without playing with the tripod legs. On really unven ground, you might adjust legs to get rough level, then fine tune with the 438.
My friend has a leveling centre post for his Manfrotto 055 tripod. I find the normal centre post plus the 438 to be a more convenient rig.
http://www.flickr.com/photos/triumphant/ for photos