A quick scan through Curt Sachs's international survey "Reallexicon der Musikinstrumente" for the ones that appear to be specifically German gives these (leaving out all the words for obscurely ingenious features of keyboard instruments):

Adiaphon
Akkordguitarre
Akkordzither (= autoharp)
Aliquodium
Aliquot-Streichflote
Alphorn
Altbomhart
Altschryari
Androide
Anemocorde
Angelic organ (= glass harmonica but earlier)
Aoline
Aolodion
Aolsharmonika
Apfelregal
Apollolyra
Apollonion
Apollonium
Armeeposaune
Aulodion
Baroxyton
Bassbomhart
Bassethorn
Bathyphon (= early contrabass clarinet)
Beckmesserharfe
Bellonion
Bissex (= early twin-neck guitar)
Bogenguitarre (= early guitar with f-holes)
Bomhart (= bombarde)
Bordunsaxhorn (= contrabass saxhorn)
Brettltgeige
Brummeisen (= Jews harp)
Brummtopf
Bumbass

and 24 letters to go. Some of those are triumphs of Victorian weirdness.