1774 Mexico 8 Reales Counterstamp Puzzler Thoughts ?

We recently acquired this interesting 8 Reales 1774 Mo FF
Counterstruck W/ G R 5 .
So far research has yielded a couple of results , thanks to my Good friends and Colleagues, Garry Charman & David Vice in the UK .
The piece was listed in the British Numismatic Journal c 1892, described as referring to an Ordnance referencing the valuation
of the Spanish Milled Dollar @ 5 shillings thus the 5 .
The G R obviously for Georgius Rex .. King George III .
Possible usage was apparently British North America, specifically Lower Canada , Quebec.
There is a second reference to the piece in a 1955 Spink Numismatic Circular as being for sale .
Any further assistance is greatly appreciated !