1771 Portrait 8 Reales With G III Counterstamp

OK so, the first obvious issue with this coin is that, in so far as I can tell, these coins were only issued from 1772 onwards. It is underweight also at 24.91 grams.

I do know that many fakes were deliberately made by the British in the late 1700's to undermine faith in the Spanish coinage. The counter-stamp I would tentatively say looks real so perhaps this is a British contemporary forgery?

Or perhaps I'm wrong, and this is simply a standard counter-stamped George III dollar. But I'm working on the assumption this is a forgery - though perhaps a contemporary and interesting one. I know there are many members with great knowledge in this area - hope to hear from some of you.
Thanks.