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.