Has anyone seen this?

There is a raised marking behind the bust on this 1911 British Sovereign.
I have spent many hours trying to identify it but to no avail.
It looks like M__M with two characters in between, possibly a T & I ?
My local coin shop says it is a genuine sovereign but hat he has never seen this mark either.
Being raised instead of stamped implies that it was part of the die that struck the coin.
Any help will be greatly appreciated.