When Did Self-Stick Airmail Envelopes First Hit The Market?

Here's a slice of postal trivia I haven't been able to date.

Shown below is a relatively common US Airmail Cover dated to 1960:



The interesting point is the reverse side shows this to be a self adhesive type cover (a first generation adaptation given the extreme discoloring of the adhesive):



Which brings me up to the question at hand: When did self adhesive (a/k/a self seal) airmail envelopes (or any envelopes for that matter) first hit the market? My example is dated 55 years ago which seems quite early for such an innovation.

As an added clue, this is the detail printed on the actual envelope, apparently a product of the US Envelope Company:

Here is a patent, filed in 1934 (there is a scanning typo of "1954" in the text at the link), granted in 1935:

http://www.google.com/patents/US2014914

Interesting. I had no idea the patent went back that far. It would be interesting to see some of the earliest self-stick (self-seal) envelopes. Does anyone have any they can show?

Thanks for the help.