From Mark Irwin - and info about this is in the Peterson Book.
Peterson quit staining the inside of their bowls around 2005. There is bare wood on the inside of the chamber and in the shank. From gateway pipes on up.
I'm going to be doing my follow up article on this as well.
Here was my first one:
Mystery: The Peterson System Stem That Time Forgot - http://pipesmagazine.com/blog/pipe-collecting/mystery-the-peterson-system-stem-that-time-forgot/