The two headstone puzzle

Walking through the cemetry of the local church earlier, I noticed that there are several graves with two headstones - one small and one large. In each case it's virtually impossible to read the inscription in the large one as it has weathered heavily. The small ones are relatively untouched by the weather and each seems to carry a simple inscription of two letters and a date - in the case of the one in the foreground here, 1803.
Graves from the mid 1800's onwards don't seem to have these small stones - does anyone have any ideas about their purpose or significance?
I also am intrigued by the pattern on the edge of the small stone - is it purely decorative or is it an artefact of chain drilling before the stone was cut from a larger piece?