These Norfolk & Western 40-foot box cars were part of their effort of rebuilding the aging single-sheathed, outside-braced box car fleet. The cars' underframe, end panels, and the basic shape were preserved, but the side panels were replaced with metal sheet. While they were in the shop, their KD brake system was replaced with the AB one.
The Norfolk & Western Historical Society has all of the diagrams and drawings available online and for purchase. Apparently the road number range 43500-43999 were a separate group within the larger rebuilt group. Kim Thurlow's N&W Portfolio of Box cars is also another good site.