I've seen this before on both pictures taken during the war and after. Think that it was probably a workshop added feature to carry more stuff. One of the CBC recording units was equipped with what looks like a combination rear step and storage box ( see photo
http://canadianmilitarypattern.com/CBCandBBC.html ) also in Wheels and Tracks Number 2 there are pictures of HUWs with something additional in this area. There is also a HUA with something stored in this location.