It’s identical on both (at least the one to the right of LBJ’s head) so more likely to be a discontinuity than shadow, well spotted.
Not in all parts of the world. I notice that the wikipedia article suggests that the sheets where hand painted before posting, not painted on the billboard, but I’ve never even seen that: billboards around here were printed even in the 70’s.
FWIW, I’ve read that when there were painted billboards in Aus, they were mostly painted by immigrant Americans.
Which is why there is a famous film from the era about the matter.
Thanks! Although looking at it again, there’s also a darker shading that extends at an angle below the “seam” on the right side of each photo. So even less likely to be a shadow, but maybe it’s some kind of horizon in the actual shot?