digitorium

from The Century Dictionary.

• noun A small portable instrument used for giving strength and flexibility to the fingers in piano-playing.
• noun It is shaped like a diminutive piano, and has a keyboard with five keys resting on strong metal springs. Also called dumb piano.

from the GNU version of the Collaborative International Dictionary of English.

• noun A small dumb keyboard used by pianists for exercising the fingers; -- called also dumb piano.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small keyboard used by pianists for exercising the fingers, without producing any sound; a dumb piano.