If memory serves me correct the DA rock carving was carved within two days of his death on July 2, 1863. DA is for Captain David Acheson of the 140th Pa. Post battle, 140th C Veterans returned to the rock. They deepened the previously carved DA and they added the 140 PV to the rock below the DA.
His family located his remains based on the carving and reinterred him back in Washington, Pa around 10 days after he was killed.
There is a recent post here on how to find his rock carving. It is the only rock carving (only the DA portion) that can be attributed to the actual battle time period. Between rock carvings, witness trees, historic buildings, the unique Gettysburg geology and of course the battle itself makes Gettysburg a fascinating place to visit. JD Petruzzi's excellent Gettysburg Guide has a chapter on rock carvings with how to find verbiage.