This overcoat is pictured in the reference books which you can find on the main page of this site and is also explanied in the Germazn language books from Jürgen kraus. The overcoat is nicely marked "daunenmantel", and dated 1914 with a stamp from the Bekleidungsamt.
This item is not cheap, but were would you be able to find another one ?
Click here to see a period photograph of the Daunenmatel
difficult to see, but dated 1914 and a BA stamp