According to the Mainichi Shinbun, an object dug up in Hanyu, Saitama in 2012 and identified as some sort of equestrian equipment turns out to actually have been a jaw harp (and I use the past tense advisedly -- it ain't gonna be byonging out anytime soon).

Reading between the lines, it seems to have been initially misidentified because similar instruments that were found previously were (a) significantly smaller — the Asahi Shinbun says that it's 14.8 cm long — and (b) not found near a shrine, so the traditional explanation of ritual use looks a bit shakier than usual. (Not at all ruled out, of course.)

It's interesting to note that there is an Ainu mouth harp called a mukkuri, although I've never heard of a metal mukkuri and there's no reason to posit a direct relationship here. (Indirect relationship, shared NE Asian heritage of shamanism? Maybe.)

The instrument has been dated to the first half of the 10th century, which makes it the oldest ever dug up in Japan, and according to the Japan Jew's Harp Association (because of course there is one) press release, the oldest dug up anywhere in the world. This is surprising, but I see no real reason to doubt them.

Finally, the Asahi Shinbun article quoted above also mentions that the jaw harp was known by the name biyabon during the Edo period (and was popular for a brief while). The Nihon Kokugo Daijiten confirms; their first citation is from ca 1825, in one of the publications of the Toen-kai (a society apparently founded by Bakin devoted to sharing weird stories and objects they'd come across):

琵琶笛、童稚訛りてビヤボンといふ、文政七年甲申の冬十月上旬より、江戸中流行す

The biwabue, mispronounced biyabon by children, was an Edo-wide fad starting from the first third of the tenth month of Bunsei 7 [1824].

So they're arguing it was originally called the "biwa-flute" and biyabon is a mispronunciation of that? I don't believe that for a second. Biyabon might be the most onomatopoeic word I've ever seen. (And I have never wanted to romanize /N/ as <ng> so much.)

Other pronunciations/names listed by the NKD and its citations: biwabon, kiyakon, Tsugaru-bue.