Genuine

There is more detail in the lower ribbon
The paper is whiter
The cancel is greasy and bleeds through
Used stamps have no gum

The gold border is not tarnished

Forgery A

There is less detail in the lower ribbon
The paper is thicker and yellower
The cancel dies not bleed through and is more modern
Used stamps may have gum

The gold border is tarnished

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Bibliography

The Work of Jean De Sperati by the British Philatelic Association 1955

Album Weeds by Rev. R. B. Earee

The Serrane Guide by Fernand Serrane

The Forged Stamps of all Countries by J. Dorn