I just got this in hand from the UK...I sent pics to Contemporary Counterfeit expert John Lorenzo, of New Jersey, and he gave me the good news...he said:
"Superb. Its listed in our new book. Give it this variety number:GNL#1792-O:Aa/R:MoFM-001......Only one other known example with an octagonal counterstamp and this example is in the Gord Nicholsof Canada Collection. His piece is debased silver over copper core and yours is a brass alloy composition.
Weight is 24.76 grams.
Note the filled-in period and no small o above M for Mexico City "
A wonderful example and a real keeper!