I'm not sure why Brennan is always singled out as the witness to which window the shots came from. He is certainly an important witness since he gives his report within a few minutes and couldn't be influenced by later reports.
We don't know what Brennan may or may not have said "within a few minutes".
But Bob Jackson saw a rifle in that window and pointed it out to Couch who also saw it.
Jackson said "I saw the rifle, or what looked like a rifle approximately half of weapon, I guess I saw". Couch said "And seeing about a foot of a rifle being - the barrel brought into the window. I saw no one in the window - just a quick 1-second glance at the barrel", and that was after Jackson said "there's the rifle", so it's not like it was an independent identification of an actual rifle. Also keep in mind that these are recollections over 4 months later after the narrative was firmly planted in the American psyche.
Conclusion: they caught a brief glimpse of something narrow sticking out of a window after hearing shots and assumed it was a rifle. Just like Marina saw part of something in a rolled and tied blanket and assumed it was a rifle. Ruth Paine tells us what Marina said to her before she mistranslated it to the police.
Mrs. PAINE - And she indicated to me that she had peered into this roll and saw a
portion of what she took to be a gun she knew her husband to have, a rifle. And I then translated this to the officers that she knew that her husband had a gun that he had stored in here.
