Undated Lincoln Memorial Cent: Struck Through Grease?

This cent turned up in change. Many of the obverse design elements including the last two date digits are missing or very weak and the portrait is mushy. I see no sign of tooling or alterations. The reverse has almost normal detail. The coin still has some mint red color and I guess it would grade AU-50 or so based on remaining luster. The date can't be read but the weight is 3.1 grams so I know it was made somewhere between 1959 and 1982.

Is this a " Struck Through Grease" type error? Are there other possible explanations? Thanks for your comments.