These forgeries of the series Mi 206-09 (Sc C1-C4) are rather poorly made, and easy to identify. While the original overprints have clear writing on the body of the plane (translation="Air Mail"??), the forgeries have only a rough pattern.
 

Forgery	Original

Forgery

Original

As for the sample above, also in this case the forgery can be easily identified with a closer look at the airplane.... Note, however, that this issue also exist with an original green overprint, a variant that was never postally used.
 

Forgery	Original