These 1929 built NP cars were part of the building rush to have cars available that could be positioned empty just outside of urban Detroit (as it existed at the time) so that these would be readily available to the auto makers for loading. These did have type "E" Evans auto loaders gear installed. The auto builders simply took suitable available and equipped boxcars on a first come basis so having cars in waiting was the best way to secure a load. However, in paying homage to the bread-and-butter traffic of the NP, end lumber doors are in place.
The cars are also 10 ft 6 inches of interior height, greater than the prior 50 ft 6000 series of 1923 (built to an SP design in which using the assembly jigs of the SP design allowed the NP cars to leapfrog up the GACC build schedule by almost a year.) The NP 6000 series had a 10 ft interior. Auto were growing longer year by year in the 1920s and this translated into a few new inches of height needed to handle this length when the auto were loaded using the various Evans gear.
One important note concerning the original builders photos: On February 17th, 1944; a directive came from the NP's Superintendent of Car Department, F G Moody that the A and B seen to the left of the side doors would be replaced by the letters L and R. (3 +1/2 inch high) The location of the letters to remain in the same location. Facing the "B" end, left and right sides are found.
For many here modeling after 1944, the L and R letting at the sides would be correct. NP maintained their equipment to a higher standard than most railroads and cars were generally repainted equipment every 7 years on the average, so one could have the original paint up to perhaps 1951. James Dick - Roseville, MN