The black line through the lower section of the art is typical for daily comic strips from the 1930s-1940s. During this period, strips were run in two different sizes (full and reduced). The full size strip was originally shot with the bottom copyright notice and made available to newspapers which ran the strip full size. The line was then drawn across the lower section of the strip (effectively creating a new bottom to the panels; with some strips, a blank piece of paper was glued across the bottom) and a second copyright notice applied for papers running a reduced size strip. This is why nothing much happened in the bottom of many 1930s-40s daily strips and why the artists made sure to sign above where the reduced size line would be inserted.No bothering with marking up the copy because once the original was shot for use in synidcation, it had no further use as far as anyone was concerned.
It's not quite as blasphemous as using the acetate animation cels for Snow White to go "sledding" across the slick hallway floor, but it's still hard to believe shortcuts like that were taken.