It is a old stone property corner. This particular stone has been etched with the initials of the respective owners on each side. It would typically be standing upright but after 60+ years it has fallen.
Reminds me of my own work last week. I was in Arlington MA trying to find a granite monument set in 1926. Finally did but it was about 6 ft from its correct position, laying on its side, apparently the victim of a snow plow. Aggravating!