@ ZTriPs .... No Offense taken. You see one thing, I see another .... I'm not totally dismissing your thought. I'm simply offering my opinion, based on experience, what I've seen and not seen.
I regard myself as a student of counterstamps and numismatics in general. Forever learning, I am. If I ever do encounter this symbol, used as the letter "C" in a counterstamp, rest assured, I'll return and eat Crow, with a Capital C. Meanwhile, omega suits me fine.
While yours is an astute catch and comparison, I doubt that our poster's coin bears the same "Grenada" mark. The Grenada stamp, along with accompanying marks, I believe, was used on silver coins in order to redenominate them. The resemblance of this particular "G" is remarkably similar to the omega sign!