I am not the best on Bowie, so i wont comment on authenticity, but from looking at it, either the item was signed on an extremely unsteady surface, and bumped. Or, it was signed very slowly and forged.
I cant tell for sure b/c of my vision and b/c of a lack of Bowie knowledge, but if its real, Bowie definetly got bumped around signing it.
I believe this to be authentic myself. Something to take into account. Prior to the "boom", 45s and LP picture sleeves when signed, were usually holding the actual vinyl records inside. No one was worried about a smooth surface or backing cardboard to write on. I believe the "bumpiness" you are seeing comes from the actual record that was still in sleeve when it was signed.
Actually the hesitations seems to collide with vinyl structure as you can see also from the ring appeared on the sleeve. The last part of the "B" is seems to follow the outside rim of the label and left part of the letter is approx near the hole. Does somebody know if this is the correct way he signed in that year?