TRIERA GRECA translated from Italian to Engels is Greek Trireme, the vessel depict on this San Marino stamp is given as a Greek triera of the 11th century. A rowing, and sailing craft with a square top-sail, which type I can’t find in any source I have or could find on the internet. A trireme has three rows of rowers, and I can’t see that on the stamp, there is only one row.
Aak to Zumbra, a Dictionary of the World’s Watercraft gives also that a “triera” is trireme.
I believe the vessel depict is a Greek merchant vessel, a commonly used vessel by the Greek merchants in medieval times, but also used as a pirate vessel. During war was she used as a transport for troops and equipment.
San Martino 1963 2li sg691.