Once the duPont additions were removed and all of the windows on the east elevation were returned to their Madison-era locations, the East elevation was once again composed almost entirely of Madison-era material (with the roof, Chinese railings and wing doors being exceptions).  So even though the duPonts made major changes to the rear of Montpelier, the fact that they reused Madison-era windows allowed the east elevation to be restored back to its Madison-era appearance using the original Madison-era materials.