If you like honset answers you need post the coin obv/rev. In full..no one can or will take the time to figure out what you have there. But there isn't any over mint marks in that series...there are repunched ones.
If you look at the entire coin, and especially the HALF DOLLAR in this case which is near the MM you'll see clear indications of die deterioration indicated in RED, which is also the case for your MM
