Ken,
According to Henderson's thee is no D. P. Mercer listed.

The closest we get is Private D.F. Mercer of Co. E, 1st Georgia Confederate Infantry (aka 36th Georgia Infantry - Villipigue's). This unit began as the 1st Georgia Battalion before getting increased to a regiment.

They lost their August Depot battle flag on July 22, 1864 in Atlanta which was sent to Ohio, was listed in the 1889 listing in the National Tribune, and has since gone missing.

Greg Biggs