This gravestone is from the 1700's in England where the law required switching from the Julian to the Gregorian Calendar systems starting in 1752. The Gregorian system was indicated by abbreviating N S (for New System) as we see on the stone.
In the medieval Latin system of common usage, people would write, "He died in the 17 Thousandth, twenty second (year of our Lord) of Anno Domini (A.D.)
Just above the stone in the picture is another flat marker. That stone appears linked to the one shown and probably contains the info on who it was that died on May 6, 1722. Since the Nowell's we can see died in 1778, this makes sense. The stone is a bridge of sorts, bridging the change of calendars used in England. Very interesting from the historical standpoint.