Nope, nickels have P starting in 1980. It looks like where the mint mark should be was struck through grease. Cool find, although I'm not sure of the value.
There was a MM initially, it's gone now for whatever reason. It's not a special case. While it could be struck through grease it is unlikely that it would just touch that area and nothing else.
I will check twice, or atleast once, from now on, If I remember. I knew they did at a certain point, and forgot.