Final answer:
The unique aspect was that the New Jersey constitution of 1776 allowed unmarried women over the age of 21 to own property, a significant step towards equal rights for women at that time.
Step-by-step explanation:
The aspect of the source that was unique among state constitutions when written was that it allowed unmarried women over the age of 21 to own property. This historical change took place in New Jersey in 1776, at the height of the American Revolution, when the fervor for liberty and egalitarianism inspired the state's constitution to extend the right to vote to unmarried women who owned property worth £50. This provision represented a significant departure from the norms of the day, as full citizenship including the right to vote was typically restricted to male property owners or taxpayers.