Antique German Stethoscope with Brass Plate

A really unusual medical stethoscope, from Germany, early 1900's, it's the first time I come across a similar model.

All ebony, in 2 pieces, what makes it unique is the metal plate inside the ear piece, to amplify the sound.

Marked on the brass plate "D.R. Patent 284522"

Lenght: 18 cm