My Easter Island Rock Sculpture

I want to share this rock sculpture I picked up today with the help of a friend. It was was on an old family property in CT. The house was built in 1876 and this was half buried in ground at the top of a granite stairway leading to the front entry. It is flat on the back side and obviously "intentionally" shaped. It was in the neighborhood of 5 to 6 hundred pounds by my best estimation. On the bottom, to our surprise, is a strange shape that protrudes from the stone. By that I mean it was not carved into, but rather the stone was chiseled away from it. The 4th picture, while not great, shows this. This stone was obviously placed where it was, so all I can say with certainty, is that it has now been moved at least twice. If any one has any input or can steer me to some one who may know what this may be about, it would be greatly appreciated. Meanwhile, he stands guard over me and my home. Thanks.