You may already know this, but I'll point it out anyway. See the white-out and the crazy paste-up lines in the original art? That's because the ad was cobbled together by chopping out the Hulk figure from the actual art for the cover to Hulk #1. They had to newly draw the center of the figure, because it had been covered by Bruce Banner in the original. I guess it's a small miracle that the originals hadn't been destroyed by then, but jeez.
It's possible that the Hulk's pinkies were added by the inker (when the cover was first inked) at Stan's direction, as it looks as if he was only meant to have 4 fingers on each hand and foot. Jack was never consistent in this though, so it's hard to say for definite.