Those two letters appear in a column used for summary accounting purposes. in otherwords, this register was used to determine the final status of each enlisted man -- transferred, discharged, died of wounds and others.
Since John B. Lanford received a promotion as Jr. 2nd Lieutenant, he left the ranks. The letters look like PA, but are probably PO, meaning Promoted to Officer. As you probably know, Special Order 141 dated Sept. 13, 1864 for his exam and promotion will be found in his service file.
John B. Lanford was captured in the action at Watkins House, March 25, 1865. We discussed that engagement at some length here a few years ago.