It is the Catalan independentist flag. Though this is true that it was first flown in the Catalan Center of Santiago de Cuba around 1903. By then, there were many Catalans in Cuba. A lot of them sided with the Cuban separatist in the fight against Spain. There is a word that the Cuban, as well as the Texan, flags inspired this one. In the lower part of the flag there are two words in our language meaning "Free Catalonia".
Answering to your other question, I must say that is not a part of any set. It is just a propaganda cinderella issued in 1919, to draw the Nations' Society (fore-father of the UN)attention on the Catalan Freedom Rights; at the moment when people's self-determination right was on the table, after WWI. It is quite common, even in full sheets, people bought and kept it.