That was awesome! I'm surprised that I ahven't heard of this instrument before, as I have a couple of friends who are organologists specialising in this specific period. I wonder if the SynthAxe people were aware of this?
Very cool, German luthierie in the 19th century was pretty experimental with this kind of stuff, extended range etc. Though instruments with this kind of key/hammer mechanism seem date as far back as the last quarter of the 18th century in England - they're actually citterns, small, mandolin-like things with metal strings which were very popular at the time.
An example of the English pianoforte 'guitar'. Note the adjustable capo as well!