A "sovereign" is the name given for a gold 1 pound coin.
The letters are mintmarks. Sovereigns were struck at various branch mints throughout the British Empire - Australia, Canada, India and South Africa as well as Britain itself.
The only mints to issue sovereigns in 1918 were the five listed by pattiewhack: Bombay, India (I mintmark) Ottawa, Canada (C) and the three Australian mints (Melbourne M, Sydney S and Perth P).
To me, yours looks like an "I".
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis