The 1st Olympic Games of the modern age, befittingly, took place at Athens in 1896.
A most beautiful set of Greek stamps were created to commemorate the games.
The 40L, 60L, 1D, 2D, 5D and 10D were counterfeited.
The forgeries were rather good and often fool many collectors as well as dealers.
Comparison of the two is sometimes required for correct identification.
Below is a comparison of a few of the genuine and forged stamps.
The genuine is the first of each pair, the second being the forgery.
As to be expected, the forgeries are a bit coarser and lack the fine detail of the originals.
The wheel of the chariot on the 60L originals is Black while the forgery consists of 3 concentric circles
On the 1D, the sky in the originals has clouds but these are absent in the forgeries
The backround shading is mostly absent in the forgeries of the 10D, while the genuine clearly shows vertical and horizontal lines.
I believe the easiest way to spot the forgeries is by noting the space between the engravers first initial and last name (imprint at bottom of stamp).
There is no space between on the originals, but quite obvious on the forgeries. See examples upper left
Note that the overprint on the 40L is also bogus. The CDS may or may not be fake.