One of the reasons it can be helpful to post photos of the entire coin in addition to the close-up is that they can help us judge the overall condition of the coin. The mintmark doesn't appear to be an OMM or RPM, so I am guessing that it is some form of corrosion. Chris
Here's the best site to see if it matches any known mint mark varieties, if it is one. http://www.coppercoins.com/advsearch.php