I am trying to figure out how to send a thank you, Phil, for your reply. The time period 1915/16 is exactly when Grandpa was in England (prior to being shipped to the trenches in France). With the Canadians, at that point, and a betrothed lady back at home - totally appropriate to having this brooch made. With a different perspective it certainly looks as though a letter could be missing, as you said. I'll try to put in a third picture showing that.
I've read now about Corke Brothers & Co - surely seems right. Thank you SO much for this info. My Grandma wore the brooch all her life, and it is her daughter, mu aunt, now 89, who asked if I could find any information about its origins.