After watching a video of a recreation of this mysterious joint, hundreds of years old but only relatively recently understood after being x-rayed, I decided to make a replica. This model has been created in OpenSCAD with a fit tolerance of zero. Printing with PLA created a model with an initially tight fit that has loosened with repeated working. I added the Japanese Kanji, which translates roughly as “Osaka Castle Main Gate Post Joint”. I can create more models with a slightly looser fit if required. The original video is here…
I'm currently trying to recreate another one called the 4-way goose-neck joint, which is referred to on the same YouTube channel above as the “Impossible Joint”.
The author hasn't provided the model origin yet.