I've always wondered about these. I saw something just like this on ebay a year or so ago. The same sort of folded-up-german-aircraft-fabric-mailed-home-during-the-war sort of thing. And recently there were two or three more on ebay. Maybe this is one of those. The backs of these pieces of fabric all seem to be about the same color, though the fronts are different.
I don't think one should assume them authentic as the trade in fake fabric is quite brazen. There are specific characteristics of authentic fabric which you can test for. Here is a run-down to get you started.
a. count the number of threads. There should be approximately 54 per inch. The easiest way to count is to shine a light through the front of the fabric and count from the back side. A photographer's lightbox is very handy for this. You will need a good magnifying glass. Lay a ruler alongside the edge and look at both through the glass. If possible, photograph what you see.
b. determine if the fabric is calendered. This is a process by which the fabric is crushed between rollers. If it would be at all helpful, visit The Aero Conservancy web site and scroll down through the German section. You will see several samples. The site is at
http://www.aeroconservancy.com
c. take as many photos of the fabric, front and back, especially any stitching. The high resolution the better, though I do not suggest you post them on the aerodrome site as they will take too lomng to load, but make them available to anyone interested in consulting on this. You can certainly send them to me and I would be happy to give you my opinion.