What is the oldest written source in this country that mentions this right?
Guyana
The first assertion of the right to freedom of association in Guyana is in the 1966 Constitution, ratified on May 16th. The right is found in Chapter II, Article 13 (“National Assembly of the Parliament of Guyana”, 1966).
1966. National Assembly of the Parliament of Guyana. May 16. https://parliament.gov.gy/new2/documents/bills/21123/statutory_instrument_guyana_independence_order_1966_no_575.pdf.