l.cutler said it best. The O over the S is a die variety and not an error. If you look close on the "S" Morgans and the "O" Morgans you will notice that the "O's" are wider then the "S's". That coin shows the wide "O" mint mark. On the "O" over "S" the "S" die was re-cut into an "O" and the die cutter followed the outline of the "S" mintmark. On the true "O" over "S" the "O" is slightly more oblonged. This coin has more of a circular "O" mintmark that has been damaged pretty bad. The marks on the O are more recent as you don't see the same patina in the cut as you do the rest of the coin. The damage was recently done at that spot. Was it ground dug?
In the past people attempted to peen over the S mintmark on the 1916 Merc into a D. Then they would artifically age it to hide the hammered marks. Even though under slight magnification it looked authentic, the "width" of the D was to skinny.
You can always send it in to have it certified. But for arguements sake in the condition that it is in would it pay off to pay the fee on a coin that would only differ in value of $20.00 - $30.00

Only to come back a New Orleans mink mark and not the San Fran the O/S was cut from?
I know I'm kind of a new guy to this forum and haven't posted for a while from moving into a new home. But I "do" have 30 years of detecting under my belt with some unbelieveable finds to show for it. I have collected and graded coins for even longer then I've hunted. I started out with a Whites 4db!!! So please don't take offense to my reply. I'm only offering experience. And when I learn to post I'll show some of what "I've" dug here in New Jersey.
Steve