Ok, it seems like this stamp issue is extremly rare :-), no one have a copy of it, well I promised you what to look for at the forgery.
Many collectors have this stamp and do not know there is a Dangerous forgery, just like the 1914 Merode and Little albert red Cross.
Her we go:

Genuine
1
The peak of 2 has the same width as the baseline
2
The F's after the digits 2 on the left and right are equal
False
1
The top of 2 is narrower than the bottom
2
The letter F after the right 2 is larger than the F after 2 on the left approx. 0.2 mm
