I have identified the R. Clark in Union POW records. He was Reuben Clarke, D/22nd Va., captured 3 May 1863 Fredericksburg [probably Chancellorsville where 22nd Va. Bn was heavily engaged]; paroled by the Army of Potomac 4 May and sent to Washington DC that same day for exchange; sent to City Point 10 May; received there 13 May for exchange. He was subsequently KIA at Gettysburg.