An even finer example of this Philadelphia-Mint rarity. When one considers the mintage and extremely low availability of an issue such as the 1871 eagle, it soon becomes apparent how undervalued it is. The same can be said for numerous S-mint issues, and the common thread to each is the lack of references to help guide collectors. Only one book has been published since Breen's Complete Encyclopedia in 1988 ('Prohibitively rare above VF') that mentions the 1871 eagle, and that is the Garrett-Guth reference. With the publication of so many references in other series over the past 20 years, there are still obviously a few unexplored areas in U.S. numismatics