In “A History of the Catholic Church in Japan from its beginnings to the early Meiji era”, Joseph Jennes refers to Christian tsuba (but also to banners, helmets, stirrups and saddles). He shows one he found in one of the books of the art historian Tei Nishimura; “Namban-bijutsu” (Tokyo 1958 西村貞、『南蛮美術』、大日本雄弁会講談社、1958).
So it seems that at least Tei Nishimura believed in Christian tsuba.
Paul V