The Steen-Nielsen Hammerspinet is an excellent example of unique and complicated engineering resulting in superb playability and excellent timbre. Approximately 400 of these were constructed in the 1960s and 70s by Steen Nielsen in Denmark. Although technically a fortepiano, it has but one string per note and is built with a "harpsichord" stop that allows it to imitate a harpsichord sound in addition to its delicate fortepiano tone. The craftsmanship is exquisite and the case is in an elegant Brazilian rosewood. There is good pianissimo to fortissimo control, and note repetition is consistent and quick. The condition of this Hammerspinet is near perfect and its tiny size will allow it to fit in even the smallest space. Overall length 46in., width 38in.
The Steen Nielsen Hammerspinet at first glance appears to be a harpsichord, but it is far from it. This instrument has many features rarely seen in piano before, and ones never seen in a piano of this high quality. Each key has only one string per note from bass to treble, as well as having all the tuning pins on the rear side of the cabinet, where normally, you would have the hitch pins for the strings. Also, the action and damper lever assembly is entirely under the keys of the piano. Here's how the action works. A normal jack is connected to the bottom of each key. The key end is connected to a bar as a pivot point and holds the key in position (there are no balance rail pins). As the key is depressed, a let-off button makes contact with a wooden rail, which then trips a pivoting hammershank towards the string. At the same time, another let-off type button depresses the back of a damper lever in the horizontal position, lifting it off the string to allow the note to sound. The weight of the action is adjusted by a screw under the rear of the key that is in contact with a spiral spring (no key leads are used). The height of each key at rest is controlled by a screw on the top of the key just behind the keytops, that comes in contact with a weighted wooden piece that is part of the piano's cabinetry. Keydip is controlled with a normal front rail adjustment.
Here is a video of a Hammerspinet in action (the Hammerspinet sounds even better in person than it does in this video):