Samuel R Caldwell listed as unknown Co 1st NE Cav. No enlistment date. Listed as Captured Sept 15, 1862 Lewis County. Sent to Gratiot then Alton sentenced to hard labor for duration of war, discharged Alton April 16, 1864. Listed as part of Porter's Band of bushwackers or Poindexter's Regt. One in prison file card says Samuel H Caldwell.

1. Samuel Caldwell-1 listed as enlisting Co D Capt Frame's Company, Snider's Battln, age 16, July 28th Lewis County by Major Smith. Note on card indicates unit broken up and most captured trying to go south.
Associated O Kemp Caldwell age 18, John Caldwell age 21, James Caldwell age 18.

2. Samuel Caldwell-2 listed as enlisting Co D Capt Edward Knight's Company Snider's Battln, North Missouri Cav Volunteers, Aug 15th 1862, Ralls County, by Maj. Snider. Age 18.
Associated Thomas Caldwell age 25, Ned Caldwell age 44, Joseph Caldwell age 33, James Caldwell age 21.

Thus I suspect that Samuel R Caldwell is one and the same as #1 above. Lots of folks captured in Sept Oct 1862 and sent to St.Louis fro NE MO PM.

Following that line of thought:
Oliver Kemp Caldwell is found in Lewis Co census as well as a James H Caldwell 3 years his junior. Also in Lewis County is a Caldwell family with a James and John yet ages are reversed with a Samuel Caldwell of the proper age.

1850 Census Lewis County Caldwell's
Name: James Caldwell
Gender: Male
Age: 10
Birth Year: abt 1840
Birthplace: Missouri
Home in 1850: District 48, Lewis, Missouri, USA
Attended School: Y
Line Number: 38
Dwelling Number: 182
Family Number: 182
Household Members:
Name Age
William Caldwell 36
Janie Caldwell 36
Wm S Caldwell 16
Mary L Caldwell 12
James Caldwell 10
Frances Caldwell 9
Saml Caldwell 7
John Caldwell 6
Ann Caldwell 4
Thomas Caldwell 1

and in 1860
Name: Samuel Caldwell
Age: 17
Birth Year: abt 1843
Gender: Male
Birth Place: Missouri
Home in 1860: Highland, Lewis, Missouri
Post Office: None
Dwelling Number: 157
Family Number: 165
Attended School: Yes
Household Members:
Name Age
Wm Y Caldwell 48
Jane Caldwell 48
James Caldwell 19
Frances Caldwell 18
Samuel Caldwell 17
John Caldwell 15
Ann M Caldwell 12
Thomas Caldwell 10
Hazletin Caldwell 9
Robert Caldwell 7
Elizabeth Caldwell 6
Mary McFadden 2

To follow Samuel Caldwell-2, Note this IS a Samuel R Caldwell. But I can't match the other Caldwells. Ther is a Cullwell family in Audrain County that is close but has associated draft records in July of 1863. The associated Caldwells are also listed as released from capture and served in US service.
Found in Ralls County Cemetary
Name: Samuel Ransom Caldwell
Birth Date: 11 Jul 1844
Death Date: 25 Jan 1882
Cemetery: Barkley Cemetery
Burial or Cremation Place: New London, Ralls County, Missouri, United States of America
Has Bio?: N
Spouse: Elizabeth S Caldwell

Could the 2 be the same I doubt given the enlistment dates.

John R