"I'm pretty sure (I wish I could find more history about it) that the the toponym "River Plate" is a relative neologism coined in the British Empire (esp. in England) perhaps as little as a century or so ago. " - You're surety is misplaced. The British have been calling it River Plate since Drake. With the barest minimum of research I counter your assertion with the fact that a book entitled "A narrative of the operations of a small British force, under the command of Brigadier-General Sir Samuel Auchmuty, employed in the reduction of Monte Video on the River Plate, A.D. 1807. By a field officer on the staff. Illustrated with a plan of the operations." - was published in 1807. See Samuel Auchmuty.