Teil des sprech horn der Grosser Feldfernsprecher 17 (Insert of the horn of the German field telephone 17)

The "bigger" FF-17 was introduced quite close to the end of WW I and apparently not so many "Grosser Feldfernsprecher 17" units have been manufactured. There was also "smaller" model of 1917 manufactured which is much more difficult to find than the "bigger" FF-17.