March 1929.
I’m only kidding, but it might as well be.
Mission either was a Gaffers & Sattler predecessor or was a Gaffers & Sattler brand. Gaffers & Sattler last manufactured as Gaffers & Sattler in 1970. Since then they have either wholely or partially been part of Armstrong, Magic Chef, Johnson Air Ease, and/or Lennox.
It’s such a convoluted history that I’m still confused myself. Nonetheless, it definitely is prior to 1970.
If I had to make an educated guess, though, I would say November 1969, which I derive from the L9.