Hello Clare, I took a look, but I couldn't dig up any more information than what you already have. Well, I was wondering about the attribution, because the sticker certainly says Kōransha, but I couldn't find any signature, so I am wondering if this really is a Kōransha piece, or if the sticker is misleading us. OR, there could be a symbol under that sticker that might help narrow it down.
The black number on the base is a museum acquisition number?