I recently, er.. have gotten the power to pause the movie on demand, it's incredible the amount of details you find!
For example, in a single frame in which Ennis reads the postcard from Jack:
1) Jack's official location in Texas is the town of Childress.
2) The postmark on the card reveals Sept.1967 as the official sending, and Jack mentions the "24th" as the day he will be passing through town, thus one can conclude that their second meeting was on September 24th, 1967.
3) The image on the postcard is also revealed as "El Capitan and Signal Peak, Wy"
4) The letter is NOT specifically addressed to Ennis, if not sent by Genera Delivery (like many of you suggested) to Ennis, and I'm sure the postmaster then forwarded it.
Well, that's just one frame, but lot's learned, I'll be posting any additional details as I find them, feel free to ask for a specific one!