Dried Adhesive.. Some other pennies were stuck to it on both sides then taken apart leaving an impression on the glue. Not a mint error of any kind.
Especially in the past, coins were attached to cards that were mailed to folks as part of sweepstakes. Basically, stick a dime to the card to cover postage to increase the likelihood that someone would subscribe to a magazine and enter into the raffle. The glue would sometimes stick to the card...sometimes to the coin. Youngsters also create projects for school and sometimes incorporate money into it. Hot glue a coin to a picture board or the like. Once the project is done, the coins are removed...but not the glue. Back into circulation they go...for folks to find and wonder about.
To me, it looks like the impression of the jaws on a pair of pliers...on both sides...the orientation is correct...
Your coin would make a good experiment for you. Soak it in acetone and see if the substance doesn't come off. Maybe use a Q-tip or wooden toothpick to ease the material off. This way, you have gained first hand knowledge and should be able to spot these in the future. A good educational experience.
