I never knew that about the interior being black.

Seems like it would have been cheaper to make it body color.
Wots this "book" that says it was all black?
IMHO, the bodies were painted green (or possibly some fleet color) and this the general part that could be done easily as a unit - doors, seat frames, interior doorposts. Not rubbed out except the custom colors MIGHT be.
THEN, parts were pre-painted in gloss black and then physically applied to the shell: windshield garnish, windshield, dash rail, window rail, window garnish, delivery pocket behind the doorposts, rear window frame/garnish.
The interior appointments that were chrome, like the dash, could be either chrome or painted black along with the parts applied. This depended more on whether the dash or brake/shift handle had defects that needed to be covered up - Ford threw away NOTHING that could be sold.
THEN the interior covering was applied. Mostly brown for 28-20 and black leatherette for 30-31. The seats matched the interior covering.
THEN the roof was put on including the ribs (plain), black muslin, padding, roof materials.
Dunno. Construction of the trucks was at best VARIABLE, even between US production facilities. Overseas even more so.
Joe K