I'm building the AA kit of the Humber Box and for whatever reason the rear hub is omitted, leaving the wheel mounting rod to suffice as a hub. All of the photos I've found of the Box are taken from the front starboard quarter, looking back, so that the details of the actual hub are very limited, but they are clearly not the same as the front. The best photo I've found is a still unacceptable view of the Box used in an episode of Foyle's War "They Fought in the Fields" but still, not good enough to use.
Can you point me toward a good photo?
You should look online for Humber Utility as well as for Humber Box, there are more photos of the Utility than the Box even though they are the same vehicle. Most photos online that come up when Utility is searched show a small rear hub or no hub at all, just a level space that shows through the center hole in the wheel. And thanks to George Moore you have now seen that there are two hubs that were used on the rear axle of the Humber Utility, the "full floating type" and the "semi-floating type".
So, I assume that the Accurate Armor wheel mouting rod that sticks through the rear wheels is supposed to suffice for a semi floating type, and the kit provides full floating types for the front wheels.
Hope this helps.