8 Reales Mexico 1792, Suspected Birmingham Issue

Firstly, hello, this is my first posting. At the weekend I bought a coin that was meant to be a genuine 8 reales minted in Mexico in 1792. I made the purchase at a small London coin fair that I attend regularly.
I know most of the dealers there including the one from whom I made the purchase. Usually there is a good amount to choose from, however that was not the case on this occasion. I have a fondness for the portrait type 8 reales and own approaching thirty examples. This coin attracted me mainly for the condition. Further more I was reluctant to leave with nothing.

Having said this I had reservations. Firstly the lettering looked wrong in places, a weak E in ET. In places the lettering seemed ill proportioned. The numbers in the date, particularly the 9 and 2 looked wrong. But strangest of all there is a dot above the mint letter M where there ought to have been a circle.
I made a number of inquires with other dealers there, asking specifically about this last particular of the mint mark. The consensus was that the coin was right, no one suggesting otherwise. One dealer told me the were numerous varieties and it was likely just one of these.

The coin was encapsulated in cardboard and cellophane so I thought it pointless to weigh it then and there. As it turns out, this precaution would have been adaquate. I think I held back on a point of manners, some of these dealers can be a bit touchy. Anyway, a lesson learned.

I weighed it once I was home and it measured 25.49 grams including capsual. Obviously an 8 real in good condition ought be very close to 27 grams. I knew it was wrong and on further inspection saw other faults.
Searches led me here and I suspect it to be a Birmingham issue counterfeit. Swamperbob seems to be the authority on the matter. It is through Swamperbob's posts that I have arrived at my suspicion. I am however, no expert. Please could you tell me what information and images I should post to clarify this issue.
My thanks, Frederickfred