The site in Itabashi was not chosen at random. In 1871, six years after Sawa's journey and three years after the Meiji Restoration toppled the old order, the new government established a gunpowder and artillery factory on the former Edo residence of the Kaga Domain. The location offered a practical advantage: access to the Shakujii River, whose current could power the hydraulic machinery needed for explosives production. This was the first explosives factory established by the Meiji government, and it was also the largest government-owned factory in Japan at the time. Initially known as the Itabashi Explosives Factory, the facility grew as Japan's military ambitions expanded. By 1936, it housed the Army Explosives Research Institute, complete with brick laboratories for materials testing, a combustion laboratory, and a ballistic tube where ammunition could be tested under real firing conditions, with shots directed into an earthwork backstop.

In 1940, the Itabashi facility became the headquarters of the Tokyo Second Army Arsenal, one of two primary weapons production complexes serving the Imperial Japanese Army during the Pacific War. The Tokyo First Army Arsenal operated on the opposite end of the capital. The Second Arsenal's reach extended far beyond Itabashi -- subsidiary plants were scattered across Japan, from Fukaya in Saitama Prefecture to Kitakyushu in Fukuoka, from Uji near Kyoto to Arao in Kumamoto. At its peak, the network formed a distributed manufacturing web spanning much of Honshu and Kyushu. But the war turned against Japan, and by October 1944 the Itabashi factory was evacuated. Most production relocated to the Fukaya subsidiary, with the new headquarters established at what is now Fukaya Daiichi High School. The Itabashi site fell silent as a weapons factory, its brick buildings and testing chambers standing empty.

After Japan's surrender, the Itabashi site underwent a transformation that its founders could never have imagined. The surviving laboratories and industrial buildings were not demolished but repurposed. The Noguchi Research Institute and RIKEN, Japan's premier scientific research organization, took control of a large portion of the grounds. The brick testing chambers that once measured the performance of artillery shells now housed physicists conducting fundamental research. The arsenal's infrastructure -- designed for precision measurement and containment of explosive forces -- turned out to be surprisingly well suited to postwar scientific work. Itabashi became one of Japan's main physics research sites. Several prewar buildings survived on the grounds, as well as on the adjacent campus of Tokyo Kasei University. In 2008, the former Second Army Corps of Engineers Building on the university premises was designated a Tangible Cultural Property of Itabashi Ward, recognizing its architectural and historical significance.

The site's survival was not guaranteed. In 2014, Asahi Kasei Real Estate drew up plans to redevelop the property. Itabashi Ward responded by assembling a modern heritage research team that studied the site's historical importance and successfully lobbied to rezone the proposed development area as a public historic park. The effort paid off in 2017, when the site received designation as a National Historic Site of Japan under the name Army Itabashi Explosives Factory Site. Today the park preserves the layers of history embedded in the landscape: the Belgian grinding wheel from Sawa Tarozaemon's 1865 journey, the brick walls of Meiji-era laboratories, the earthwork backstops of the ballistic testing range, and the quiet grounds where physicists once worked in buildings designed for war. It is a place where Japan's transition from feudal isolation to industrial military power to postwar scientific achievement can be traced in brick, stone, and iron.