NGC -
Every United States coin features the date it was issued on the obverse side. The mintmark, on the other hand, is virtually always on the reverse, although there are some notable exceptions — particularly in the modern era.Read More: Counterfeit Detection SeriesBy placing the mintmark on the reverse, the US Mint was able to economize its use of dies—while a new obverse die needed to be used each year, the same reverse die could be used by a mint for multiple years.
Counterfeiters have adopted these same efficiencies and, as a result, the same spurious reverse is used to produce fakes of a variety of dates. This counterfeit 1879 Double Eagle, for example, features a reverse that was also used for illegitimate 1894, 1904 and 1907 Double Eagles.
Counterfeit 1879 Double Eagle
The repeated use of the same spurious reverse can often make identification of forgeries easier because they all share the same diagnostics. This reverse has a triangular depression below the three arrows and if you see this flaw on any 1879, 1894, 1904 or 1907 (or other years, for that matter) you should instantly become suspicious. This piece also exhibits tool marks on the obverse below the E and T of LIBERTY—another indication that it is not genuine.

Identification of repeat depressions is a key method used to detect counterfeits. It is simply not possible for two coins to show the same "abrasions" (called "depressions" when they are on a counterfeit) in the same exact places. In the case of this forgery, knowledge of a single triangular depression on the reverse can help you identify at least four dates of counterfeit Double Eagles.