Hi Roger, Ive just had this response from John at Tinsoldier,
Hi Steve: I think not. None of my sources mention such a patch. That doesn’t rule it out, but J. Hefter did not mention it and he is probably the best source on the Mexican Army of that period. Thanks and good luck with your project. John McEwan
So it seems I shall go with no patch, after all that! many thanks for your input into this - I shall post pics soon!