Mary Anne,
Joanne Chiles Eakin painstakingly sifted through the microfilm files of the Union prison records in the National Archives and Records Administration (NARA), specifically for the St. Louis area military prisons in both downtown St. Louis and the prison a few miles away up the Mississippi River in Alton, Illinois. Ms. Eakin placed in her book only the records for any prisoner with Missouri connections. In essence, what she was using were the Union prison clerk's ledger entries for each prisoner, although many such records are evidently missing or failed to reach the archives after the war.

The entry for Robert O. Green says: he was a private, from Saline County (no military unit given), captured at Milford, Mo on 19 December 1861, and eventually sent to the Alton, Illinois prison. The remarks section is blank, and, therefore gives no further disposition of this prisoner.

As you probably realize, Private Green was one of about 1,500 new recruits for General Price's southern Missouri army who assembled first in Saline County from a number of surrounding counties, then set off under overall command of a Colonel Robinson, if I recall correctly from my reading, to join Price's forces. This group was overwhelmed by a Union force at Kirkpatrick's Mill or Milford along the Blackwater River in northeast Johnson County on 19 December. The Union cavalry penned the larger southern force against the river, and, since they were new recruits poorly armed and trained, they prudently surrendered. That night they suffered terribly in a sleet storm, and had other mishaps before they reached Alton.

The Union military leased the former Illinois State penitentiary in order to handle this large influx of prisoners-of-war. Sadly, the old stone prison was condemned in the 1850's because of flooding and poor sanitation, problems that the Yankees corrected only somewhat while they used the facility throughout the war.

Ms. Eakin published her book herself in Independence, MO in 1995, and the full title of her landmark work is "Missouri Prisoners of War From Gratiot Street Prison & Myrtle Street Prison, St. Louis, Mo. and Alton Prison, Alton, Illinois Including Citizens, Confederates, Bushwhackers and Guerrillas," but those of us who refer to her book simply call it "Missouri POW's," or similar abbreviation. There are no footnotes, as the entire work is the copied entries from the various prison ledgers as the Union clerks kept them.

I hope that helps.
Bruce Nichols