Taishôgoto describes it as a "little old ladies'" instrument, in terms a bit like the "zithers" that were a craze in the US in the late '40s. A brief description of how it's played(?). It's described as working "like an autoharp" but appears to be more on the order of a mountain dulcimer with keys, as you described it. Not really a "traditional" instrument, but it's apparently been popular in Japan since the 1920s.

"Japan has its equivalent of the German Wandervogellaute from the beginning of the 20th century. It was created with the same combination of nostalgia and lack of historical accuracy. Around 1920 in Nagoya, Morita Gorô created a new version of the 19th century nigenkin or yakumogoto, a two-stringed koto, on which one pushes down numbered buttons above its metal strings like on a Western autoharp. It was named taishôgoto, and became a populair amateur pastime in the provinces. This is still the case today, as all over Japan groups of mainly middle aged women play Japanese as well as Western popular songs on their often electrically amplified taishôgotos. The right wrist rests on the instrument and the strings are plucked with a plectrum. Nowadays the usual size instrument has six strings, though the melody is played on just one set of strings tuned in unisons or octaves. There are at least two sizes, in different pitches. Tremolo is used a lot, and this gives a group of taishôgoto a sound resembling that of a mandolin orchestra. I have personally had the pleasure of playing lute with such a group in renderings of Yesterday by The Beatles and the Japanese tear jerker Kageo Shitaite."

Posted by a "David van Ooijen," with a nice picture of David (on lute) playing with a couple of old ladies, and a picture of one style of the instrument.

Numerous Google hits all seem to be in Japanese and/or German. A Yamaha(?) page in Japanese appears to offer several models, but I couldn't read enough to be sure if that's what it was about.

Chrysanthemum, at No. 16 on the page, has a sound clip. Sakura: Japanese popular song. Performed my Madre Magdalena Kanno (taishogoto) in rehearsal. Santa Cruz, Bolivia. March 29, 2001. 2.87 MB MP3. Unfortunately the taishogoto is somewhat overwhelmed by the accompanying whistle. The "rehearsal" quality is apparent.

A surprising number of German articles, but they seem to be mostly announcements of concerts.

Perhaps a helpful Yamaha rep would be your best bet for getting translated playing help?

John