1943 Merc

In the photo's the discoloration has a greenish appearance, this could be PVC contamination.

you could give pure acetone and try if its PVC it will be gone in seconds if it remains as it is then its something inorganic and the solutions become more limited, a dip in MS-70 would more than likely remove whats there but that stuff is touchy a second too long and the luster is gone and so is the value of the coin.

I would say it is AU-55 to AU-58 at best. The bottom bands are not fully split and that leads me to believe that there is a slight bit of rub. Yes, the middle bands are not split which is very common among mercs and is indicative of a weak strike. However, when the bottom bands are not split on a higher grade coin it can also mean that it has circulated for a little while. Besides, if I'm wrong and this is a mint state coin, it should be easy to find an example with fully split bottom (and middle) bands for the 3rd most common date in the series.

Please don't take my comments the wrong way. This is a nice coin. I just hope you didn't pay too much for it.