Missouri SOS file has an A. P(R) that may match your fella including the coming home in early 1863.

Here are the links in chronological order.

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s903/1525.tif&Fln=S162833.pdf

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s906/1917.tif&Fln=S162832.pdf

The Adj General file index has his transfer to St. Louis listed:

Howard, Adam R. Saint Louis St. Louis Received of John Parish, the following prisoner: Adam R. Howard. Isaac Gannett. 12-30-1864 F1344