Here's another example of Jack drawing Spider-man without the spider logo on the chest. One wonders if Jack simply forgot to add that to the costume, or maybe Jack figured someone would add it during the inking phase so he didn't bother with it. Also notice Jack doesn't connect the weblines on the mask to the eyes again, just like the Marvelmania piece yesterday. I suspect this was simply Jack giving his interpretation of the character; he may never have studied the costume closely enough to notice that aspect of it; or after creating thousands of costume designs in his career, he may not have remembered the exact details of the costume. I also wonder if maybe those elements may have been part of his original Spider-man costume design from 1963, and Jack decided to make the character more similar to his original vision.