On October 7, 2019, about 0830 eastern daylight time, a Honda, HA-420, H166HJ, was substantially damaged when it was involved in an accident near Charleston, South Carolina. The airline transport pilot and four passengers were not injured. The airplane was operated by Pro by Air, as a Title 14 Code of Federal Regulations Part 91 executive/corporate flight. The pilot stated that, during the landing approach, he moved the landing gear handle to the DOWN position. The main landing gear indicator was green, but the nose landing gear indicator was yellow; followed by an unsafe audible alarm indicating that the gear was unsafe. The pilot aborted the approach and requested a tower flyby, during which the tower reported that the, “doors did not look right.” The pilot attempted to cycle the gear twice, but was not able to resolve the issue. The pilot declared an emergency and performed the emergency gear extension checklist, after which the gear lights indicated that the two main landing gear were extended and locked; however, the nose landing gear indicator light was red. After performing the emergency gear extension, the pilot received a warning message about the gear doors. The pilot performed a normal approach and landing and the airplane slid to a complete stop on the runway. The pilot and all passengers deplaned on the runway and waited for emergency services. An examination of the airplane by a Federal Aviation Administration inspector revealed substantial damage to the fuselage. Further examination revealed that the pressure vessel was damaged as a result of the gear-up landing. The airplane was recovered to a hangar for further examination by the FAA and representatives of the Honda Aircraft Company. A towbar adapter lug was discovered inside of the nose wheel axle assembly. The towbar adapter lug was removed, and the length and width dimensions of the adapter lug was documented along with its part number (P228). The adapter lug was compared to the chart of the original equipment manufacturer (OEM), Brackett Aircraft Company Inc. The adapter lug was not a Honda Aircraft part, nor was it the OEM specified lug for the accident airplane. A witness mark was discovered on the top surface of the door and at the bottom of the nose landing gear bay. The clearance between the nose landing gear door and the nose landing gear axle without the discovered adapter lug inserted measured approximately 11/16-inch. The adapter lug was reinserted into the nose wheel gear axle assembly with approximately 1.5 inches of the adapter lug extending from the axle assembly. There was a considerable negative clearance between the fuselage and the adapter lug. A representative for the fixed based operator (FBO) who performed ground handling on the airplane stated that they use pin adapters when connecting the nose wheel to the towbar, because the wheel hubs are too small for the universal towbar hookup.