February 28, 1938 was the start date. The rate from Canada was 6c for the first half ounce.
Letters went surface to Great Britain, and air mail thereafter.

The 1-1/2d rate was
from Great Britain, out to the Empire. If 1-1/2d was equal to 3c Canadian, then Canadians were paying an extra 3c to get a letter to GB on the water. I bet they were grumpy about that.
Any of our friends who are devotees of collecting the Great White North have a different take on this?