The Siemens Model M-22, carbon hearing aid was made by Siemens & Halske A. G. of Germany around 1925 (engraved on the back side of the microphone) and was marketed in America under the DUOTONE name by The American Phonophor Corporation of New York.
The double carbon amplifier microphones measure 4¾" x 2⅜" x 7/16" thick. The front view sports two decorative "snowflake" designed ports, and the base has a small swiveling leg for standing on a flat surface. The leather tab is provided so that the hearing device could be attached to a shirt button.
The receiver microphone is attached to both an "ON/OFF" switch and the earpiece via two coiled wires that have a two-pronged end for battery attachment. Please note that the battery is not functional nor does it have the correct ports for receiving the wire prongs...but it came with the case and fits quite nicely!!
The condition is very good with wear commensurate with a well-loved antique.
WIKIPEDIA has some very inetersting info on this style of hearing aid which was produced from the early 1900s till the late 1930s.