It looks like a natural occurring anomaly to me. I dont see the marks or indications I would expect to see on it - from the images.
Even though it was found in a plowed field, when the material which would become this stone was formed or after, it could have had these circular depression cut into the stone some other way than a native american nutting stone.
Millions of years ago weathering and water movement could have caused a hard small pebble to bounce around in this depression, gradually making the depression larger.