Good observation samwise. I’m guessing that this is quite intentional on the part of the late Marit Allen (costume designer). Hats are mentioned only a few times in the short story and always specifically to make a point, as Annie is so spare in her writing. One of these few mentions of hats is during the 1967 reunion scene when she describes Ennis’ first view of Jack after four years:
Late in the afternoon, thunder growling, that same old green pickup rolled in and he saw Jack get out of the truck, beat up Resistol tilted back. A hot jolt scalded Ennis . . .
It seems this same hat, or at least the same style hat, is significant in Ennis’ image of Jack. Like so many aspects of the film it may or may not be “real” but the image Ennis maintains of Jack in his head is what is significant. In the movie reunion Jack's NOT wearing the same black hat you spotted in this later fishin' trip. The reappearance of this black hat in multiple movie scenes was probably intended to reinforce the idea that Ennis’s view of Jack was an unchanging or static one. (also it makes Jake look so damn sexy!) JMHO