1c rate to Canada, 1958

All, I recently acquired this cover for my Presidential series exhibit. The flap isn't sealed, so I assume that this is a printed matter rate. While I have a few 2c Presidential covers sent to the US, this is the first time I've seen a cover with only 1c. None of my 2c covers shows a year in the postmark, so I'm wondering whether the printed matter rate went from 1c to 2c at some point after 1958. Or perhaps this 1c rate was for Canada? Any thoughts would be appreciated. Thanks, John