This is a piece of German aircraft fabric from the rudder or fin of Fokker D.VII (Alb.) D. 6852/18.  It measures 3-3/4 inches by 1-5/8 inches. The rudder and fin were typically painted white and this paint has aged over time and with accumulated grime. The cross-section was photographed by me through the lens of a microscope; it shows from top to bottom the paint layer over a dope layer over thread. The postcard showing the side of the aircraft is original and accompanies this fabric; photos of the aircraft from front and above were copied from the internet. The last image measures the number of threads per inch as 49.