From the author's Atlantic Neptune.
Third state according to LC Maps of North America, 1750-1789, entry 1595.
The charting has been continued to the north and east of the Savannah River, and an inset "Plan of the siege of Savannah, and the defeat of the French and Rebels on the 9th Oct 1779, by His Majesty's forces under the command of Major General Augustine Prevost. Surveyed by John Wilson A. Engineer" has been added to the lower right corner.
The physical item is not available at the Boston Public Library.