1. The last "9" is slightly smaller than the first
2. The inner opening of the "0" is wider on the forgery
3. The distance between the "0" and the last "9" is larger than between the rest of the numbers
4. The bottom opening of the last "9" is larger on the forgery

I am not sure whether the right side picture really shows an original overprint. There are numerous deviations on these overprints and none of my stamps seem to be quite equal....

Forgery	Original?

Source:
- Billig's grosses Handbuch der Faelschungen, Part 34, Vienna, 1937