Ok first would you follow up this idea by stating where exactly is it that you think Brennan saw the man - the man's specific location.
I believe that Brennan saw the 175 pound man who was dressed in light colored khaki clothing standing behind the the wide open window at the west end of the sixth floor. Both of the windows in the set of windows at the west end were wide open at the time of the shooting. I believe the 175 pound man with the hunting rifle ( as described by Arnold Rowland) was standing behind the east window ( right hand) of the set of windows.
Brennan said that the man was standing up and using the side of the window to steady the rifle as he aimed it out of the window. I believe that the man used the center dividing mullion between the set of windows as the steady rest.
Arnold Rowland saw the man behind the window at the west end of the sixth floor, but the man was further back in the room and away from the window, when Rowland saw him.
Howard Brennan could not have seen a STANDING man aiming a rifle out of the window behind the so called "Sniper's Nest" window.... because Brennan said the man had the muzzle of the rifle outside of the window....That would not have been possible because the S N window was not open far enough. The horizontal sash at the bottom of the half open window would have prevented a standing 5' 10" man from sicking the muzzle out of the window .....
Brennan's affidavit of 11 / 22 /63 clearly describes a man who was NOT Lee Oswald.....