HR D.39 Brake Van

HR D.39 4-wheel Brake Van from a "Lochgorm" kit.

I plan to model fully the interior of this Brake Van. I've made a start by double-skinning the outer ends so the glazing will slip between the inner and outer walls. I plan to do the same to the sides, though at the moment I'm not certain about the construction of the van. Did it, as suggested by the drawing accompanying the kit, have a double skin, being boarded over inside or did it have a framework that was visible inside as is the case with many brake vans I've seen drawings and indeed photos of?

I've soldered the ducket sides in place but I'm not at all convinced about the way they protrude inside the van and extend all the way to the roof. I've tentatively drawn an outline in pencil on the ducket side to suggest how I think they might more likely have been. At the moment I'm not sure how to proceed. I know that information is scarse but can anyone help with any detail of the interior?

It's a curious little van as it has four doors, I can't imagine why it needs so many.