Since the ink of the letter is showing through the thin paper, I've reversed the image and adjusted it some to try to make out the date written at the top.

I know it's difficult to read with the stamp there, but when I hold the letter up to the light I can make out Brantford Feb 15/44
It does look like a 17 on the image, but under light, the top horizontal stroke comes out.
So I'm now thinking the pencil notion on the lower left is more probably an receiving date.