August 13, 1951: Comic magazine

A prescient strip. Here's the article from inside that issue.

(Also, it's another turnabout chase strip.)

December 21, 1950: Tick tock

I've actually done this thing, with the hands, in real life.  I believe the joke here, which is a little subtle, is that Charlie Brown doesn't know how to tell time.

This strip is notable for, I believe, being the origins of Charlie Brown's distinctive zig-zag "Crayola" shirt.  (hangashore pointed it out in the Metafilter thread first.)