"That is a chain of galley slaves, on the way to the galleys by force of the king's orders." "How by force?" asked Don Quixote; "is it possible that the king uses force against anyone?" "I do not say that," answered Sancho, "but that these are people condemned for their crimes to serve by force in the king's galleys."
Source: wiktionary
The "galley slaves" in those days were the genuine article, and by the "chains that bound them" were no doubt the original members of the union.