The Croatian post gives:
Bark (Barque) – Barques dominated among the 19th century merchant sailing ships on the Adriatic. These were mostly three-masted ships. At the end of the 19th century, large sailing ships finally lost the battle with the steamships on all the world seas and oceans, on the Adriatic too.
The new shipbuilding centres have taken over precedence by introducing new ideas and technologies.
The name barque is around the world used for all kind of craft, from dugout canoe, fishing boat and lighter to a large 5 masted vessel.
The vessels barque rigged we mostly known today, and seen on the stamp, have on the aftermast (mizzen mast) a spanker and a gafftopsail, while the other masts carry square sails.
More info on the barque you can find:
http://en.wikipedia.org/wiki/Barque
The stamp issued by Grenada in 1989 gives that on the stamp is depict a French Merchantman, depict is a sailing vessel barque rigged, the vessel is not identified till now. (Stanley Gibbons gives a full-rigged vessel but that is wrong.)
Croatia 1998 2.k65 sg559, scott?
Grenada 1998 $1 sg2019, scott1731
St Kitts 1990 $1.20 sg321, scott303
Somalia Republic 1998 500 SH. SO. sg?, scott?