The U.S. Philippines series contains a small number of important rarities, including the 1906-S Peso, the Proof 1907 Peso, and the 1903-S 50 Centavos. Among those three, only the 1906-S Peso is really collectible, so it has risen to the top of most collectors' want lists.
Why is the 1906-S Peso so rare? According to Lyman Allen, the man who literally wrote the book on U.S. Philippines coins, "At the time, nearly all 1906-S Pesos were held back for use due to the high silver content and the rapidly rising price of silver. Most of those held by the treasury, along with earlier dates, were exported to the U.S. for recoinage into the reduced size and weight pieces which followed in 1907." From this, we gather that most of the 1906-S Pesos were destroyed and that those that survive today somehow made their way into circulation and numismatic collections. The lack of Gem examples of this date confirms this theory. In fact, the finest certified examples known today top out at only MS62 (two of which are illustrated here).
Counterfeiters gravitated to this date because of its rarity and high value. As a result, collectors must beware of coins with altered dates and outright counterfeits (both cast and struck). U.S. Philippines expert, Ray Czahor, has published several characteristics of genuine 1906-S Pesos. These include the weight (26.9 to 27.0 grams), the shape of the top inside loop of the 0 of the date, a particular die break, etc. Because not all 1906-S Pesos share these characteristics, certification is a must.