Wonderful
I admire these series. It's incredible how such simple yet superneat drawings can tell such fluent and hilarious stories. You are a genius with the medium! Hooray for the music as well, that's a lot of variations on the same few polkas :-).
One thing that would make this absolutely perfect, is adjusting the shadow on the pen when it moves. Making the light always come from the same direction, if that makes sense to you. Now it still looks like a pic moving over the screen. Keeping the sense of the direction of the light, would make it more "real". If you're really daring, you can try to find a way to construct the texture of the paper in the same sense too.
But those things are really the most minor detail, this animation is perfect in every sense.