France 1920's Overprints Help

The stamps below are often forged. Those shown are certified genuine.
Most forgeries are very easy to spot, a few are not.

Question - any publicized information on the genuine ones & what to look for. Comparison against the genuine will catch 95% of them.

Thanking you in advance - I don't need photos of forgeries - I have lots of them. Today alone I notified 2 stamp collecting sites to remove obvious copies being sold by "members" for large sums.

I recommend "Fakes and Forgeries of 20th Century French Postage Stamps" by R. G. Gethin.

Very well done, in A4 format. Superb graphics.

Below is a sample page showing some of the forgeries of one of your stamps:



I am very interested in good quality images of these forgeries.