Their was an article concerning this recently in one of the Krause publications (I do not remeber if it was Numismatic News, Bank Note Reporter, or World Coin News) but it showed a wide variety of notes with different counterstamps on them.

This pratice is similiar to what was done with silver coins many years ago in Asia to insure that they were not counter fit.

It appears that our "new and improved" security designs to prevent counterfitting are not enough to deter the counterfitters.

Look for a new series of notes starting sometime next year as a result