As pictured, it shows the sliding cover (with the tri-color IBM logo) which I assume moves to fill in the gap when using the smaller-sized battery. The unit that arrived does not have this cover and I was wondering if there's some other function that it might provide: Here's what mine looks like:
http://www.kawakami-ca.com/ibm_x4x/img_8786m.jpg (640 x 480; 100KB)
I'm trying to decide whether or not to return the unit. Functionally, it seems to work just fine without the slider.
edit: I've checked the X41 Tablet HMM and this piece is apparently called an "adjuster" and has a separate FRU

edit2: Corrected "Ultrabase" to "dock"