In order to save time, many comics illustrators put an "x" in an area to show the inker where to spot blacks, but Jack tended to fill in all the shadows and dark areas with graphite. Notice the texture to his pencil strokes, sometimes changing direction, creating a patchwork that leads the eye all over the figure. At times, Jack uses this technique to emphasize the anatomy -- giving the image a three-dimensional quality. This kind of subtlety was rarely captured in the final inked drawings published in 20th century comic books because the inkers tended to go in with a brush dipped in India ink, and all that fine linework was filled in as a solid black.
Zooming in for a close-up of the image shows how important the quality of the original penciled image was to Jack -- he could have quickly sketched out the basic shape of the character, but Jack wasn't satisfied with a piece until he fleshed out the illustration completely; even though he had to spend extra time adding extra lines for shading -- many of which would not appear in the final published version -- Jack was driven to make the final penciled image the best that he was capable of producing.
Nowadays with digital scanning and photoshop, a piece like this would be considered suitable for publishing without inks added.