In the late 1980's - early 90's Muhammad Ali signed 3 or 4 times at the Atlanta show. These were big shows back then. I saw him sign 2 posters with my own eyes. The posters never left my possession. Seven or eight years later when the authenticating craze hit, I had the posters authenticated. Both failed in their opinion! Recall something about in their opinion the slant and pressure wasn't right and something about matching their exemplars.
I'd get a different opinion. Sam Majors