D.E.: You are quite right to argue the point, and it high lights the danger of stating a firm conclusion based on some research, only to find out that there is always an exception to the rule- and this was one of them, my apologies!!

I would like to learn the source of your information for my own education however, now regarding the use of diagonally installed wooden planking on which to install the iron sheeting, I submit that this was probably the choice of the designer who may have had previous experience in ship-building (or house building!) and he might have felt that the wood would take the vertical loading forces better if the plate was secured to an angled base?