Ooh, I think this could be a very special find. It looks just like some transitionals that I saw in a Robert Block essay (totally out of round with a very crude, twisted pontil). In the article, it said that that hundreds were found in the 1960s in a Shanghai garbage dump, but that they were made much earlier (like 1920s or 1930s). Shiroaiko (a collector in Japan) says that she owns a few, and thinks they were made in Japan in the 1910s to the 1920s.
I bet in the future we'll find out more information about these guys, but they're pretty mysterious. I've heard they are also pretty rare, (seeing one of them is almost like seeing a ghost).