"had been carried on by the Assiento’s "

An Assiento was a licence granted by Spain to other European nations to sell slaves in Spanish colonies. At the time, Spain had no territories of her own in the slave regions of Africa, and so her plantations relied on supply by Portuguese and later British and Dutch traders. Assientos were issued from 1543, but the British arrangement did not come into force until 1713, making this reference something of an anachronism.