The week of September 13-18 is missing from gocomics.com. It's just completely gone; the strip browse sequence goes directly from the Sunday strip of September 12 to the following Sunday.
This is a good strip to contemplate how Snoopy's design has changed so far.
He began as a very puppyish dog, much smaller than any of the other characters. While very cute, he looked almost like a piece of clip-art. Many strips these days use images of their characters in various stock poses, but not Peanuts. Schulz gradually began loosening up the design of the characters. Snoopy is the character that would develop into the loosest, and although he's not there yet, he's a lot more flexible here than he was in those first comics.
Snoopy becomes quite thin (especially when standing upright) before expanding into the "balloon animal" shape of later strips. The drawings of him in the first panels are particularly engaging. The wide smile is a distinctive mark of classic Snoopy. I notice in panels six and eight, where you can't see his mouth at all, his snout looks a little thicker than in the other strips.