If you skip to 5 min on the video you can see some of the machine they use to stamp the pipe in this video.
http://www.youtube.com/watch?v=Rs9Vjvu5xdM
Here's a video of a guy doing it by hand, you can't really see the stamp until the end of the video, camera guy isn't very good.
http://www.youtube.com/watch?v=3-UjjIUBJ5A