Current best guess: http://www.museum.osaka-u.ac.jp/en/specimen/2.html

Parent page: http://heritageofjapan.wordpress.com/following-the-trail-of-tumuli/types-of-tumuli-and-haniwa-cylinders/what-sort-of-tomb-treasures-lie-within-the-ancient-burial-mounds/types-of-bronze-mirrors/
Japanese blog post, I can't understand much but the last sentence says it was found in a royal tomb along with a spindle whorl, proper translation would be appreciated: http://blog.goo.ne.jp/tommz_1938/e/c0cfcbaa47fac51cd183fc543da41dbb
Another Japanese blog post: http://book.geocities.jp/piezo_comic/yama3.html

Other links:
http://www.ginacolliasuzuki.com/author/telling-the-time-in-17th19th-century-japan.html
http://www.onmarkproductions.com/html/12-zodiac.shtml
http://factsanddetails.com/japan/cat18/sub114/item608.html
https://en.wikipedia.org/wiki/Circled_dot
https://en.wikipedia.org/wiki/Bronze_mirror
https://en.wikipedia.org/wiki/Shinju-kyo
http://hollowplanet.blogspot.com/2011/08/ancient-chinese-cosmology.html
http://heritageofjapan.wordpress.com/yayoi-era-yields-up-rice/the-advent-of-agriculture-and-the-rice-revolution/when-japan-entered-the-iron-age/treasure-bronze-bells-and-magical-mirrors/