I'm not actually sure if the doorknob is truly Japanese or not, but was proceeding under the assumption that this is a known thing. This might be a bad assumption.
To be frank, the best evidence of his residence in Japan based on what we have so far is probably the pill-sheets. Moon runes all over that shit. Getting someone to smuggle specifically Japanese pills (assuming controlled substances) seems like a lot of effort to go through, but if he got them while there on a trip and brought them back that wouldn't be out of the realm of possibility. That said I can't read moon-runes so I'm not sure if those are just OTC pills or actually what he claims them to be.