I found this set, at one time, in an antique Louis Lange catalog from the 1930's. It showed the little pond WITH the two celluloid pieces - a swan and a fish. I REALLY wish I hadn't sold that catalog, at least, without copying some of the pages. Anyway, even though it's not marked, I can guarantee that this piece is original and authentic.
It is small, measuring 6" by 6". The interior is 4" x 4". The first photo shows the piece with about 1/4" of water and the two pieces floating. It is such a rare and unusual German piece!
They were the inventors of Putz villages - the village scenes that people display under their trees each year at Christmas. Putz means plaster, and most of the early pieces were made from composition - which includes, in large part, plaster!
Anyway, this piece would be really great for an antique doll house, or a Christmas Putz village.