Ron Guth: Finding an 1878 7 Tail Feather Reverse of 1879 Silver Dollar requires counting tail feathers and determining the contour of the eagle's breast. Fortunately, these actions are easy. 1878 Silver Dollars are found with either 7 Tail Feathers, 8 Tail Feathers or some combination (known as & over 8 varieties). Some 7 Tail Feather varieties show traces of underlying feathers, but they are not true 7 over 8 Tail Feathers. Determining the type of reverse requires a look at the eagle's breast. If the breast is flat, it's a Reverse of 1878; if the breast is rounded, it's a Reverse of 1879. It's worth a look because the 7TF Reverse of 1879 is half as common (or twice as scarce) as the 7TF Reverse of 1878.
The 1878 7TF Reverse of 1879 is a relatively common variety in grades up to and including MS-64. In MS-65, the population* is still above 300 coins, but high collector demand means there is a lot of competition for these coins. The finest examples are six MS66's. Prooflike examples represent less than a tenth of the total population and Deep Mirror Prooflikes are twice as rare as regular Prooflikes.
Sources and/or recommended reading:
*PCGS Population Report as of September 2011
Sanderson Family Collection - Heritage 1/2009:4934 - Heritage 8/2011:7303, $16,100