Tax stamp?
Start at
http://www.stampworld.com/en/stamps/Greece/ ...
On the right side, see the drop-down-box labeled "Cat." (category).
Select [ Postal Tax Stamp ]
Then, find the drop-down-box labeled "Denom." (denomination)
Select [ 5 ]
Select [ Search ]
Then, wait patiently for someone who knows, because that did not work

Cheers,
/s/ ikeyPikey