I don't either have materials to determine the nature of the parts but the engine nacelles on the G4M aircraft has similarlly shaped cross sections. This could be upside down.
From a brief look over on the parts I too recognized the quality of riveting which is far from satisfactory. My guess are;
1) under a severe wartime pressure unskilled junior labor including school boys and girs were employed in the factory to build the airplanes, filling the vacancies created after skilled workers (above 20 YO) were drafted to the war fronts. It seemed there were no system of "reserved occupation" existed in Japan and the draft office had paid NO attention to preserving the skilled employee of the aircraft factories. The Army vacuumed all male person suitable to serve as per regulation very evenly no matter what he was a university professor or a last son from a poor peasant family. They were treated equally and very harshly without regarding one's previous job and/or ability was. The above was an well-cited example of the existence of the lack of co-ordination between any of the levels and offices and ministries including the service in wartime Japan.
2) or the parts was hastily repaired at "Koku-sho" an aircraft repair/maintenance station established at some of the front air bases, quite possibly under adverse working conditions.
Source; various academic books including "学歴・階級・軍隊" (Gakureki, Kaikyu, Gun-tai) Rieko Takada, Chuko-Shinsho, July 2008, "Aces, Erks and Backroom Boys" Edward Smithiies, Cassel, 2002 and "Zero-Sen" Horikoshi Okumiya, PHP, Dec. 2000 (a full version including appendix all in Japanese language).