Here's a history of the 21st (Carter's). Found Euclid but could not find any record of Benjamin or of any unit commanded by a Col. Duff.
21st (NATHAN W. CARTER'S)
TENNESSEE CAVALRY REGIMENT
Also known as as Scouts, and as Wheeler's Scouts
Organized September, 1864; never officially numbered by Adjutant and Inspector General's Office, but known in field as 21st; February, 1865, merged with 14th (Neely's), 15th Consolidated (Stewart's-Logwood's) and 22nd (Nixon's) to form Nixon's Consolidated Tennessee Cavalry; paroled at Gainesville, Alabama, May, 1865.
This regiment is not to be confused with James E. Carter's 1st Tennessee Cavalry Regiment, although both of them were at times referred to as Carter's Regiment. The nucleus of the organization was Captain Nathan W. Carter's Company of Douglass's Battalion. This company was organized about September, 1862 at LaVergne, Rutherford County. When Douglass's Battalion was merged into the 11th (Holman's) Tennessee Cavalry in February, 1863, Carter's Company became an Independent Company of Scouts for Major General Joseph Wheeler's Cavalry Corps.
About July, 1863, Carter was authorized by General Wheeler to raise a battalion or regiment in Middle Tennessee. One company, Captain Lemuel G. Oliver's, was organized at Chattanooga July 28, 1863, and four other companies were ready to be mustered when the legality of the authorization was questioned, and no further steps toward organization were taken until the Adjutant and Inspector General's Office finally gave its approval on February 12, 1864. In the meantime Carter's and Oliver's Companies served as scouts for Wheeler's Corps. The regimental organization was completed in September, 1864, totaling about 400 men.
Colonel-Nathan W. Carter
Lieutenant Colonel-Robert Withers
Major-Robert H. Dudley
Both Withers and Dudley had seen previous service in 1st (Maney's-Feild's) Tennessee Infantry.
No muster rolls for the regiment were found. Muster rolls are available for Carter's and Oliver's Companies prior to the organization of the regiment. Captain Robert L. Morris, of Co. "F" gave a partial roll from memory of his company in Lindsley's Annals. From prisoner of war records, it seems that the company organization of the regiment was as follows:
CAPTAINs-Originally Carter's Scouts, Co. "A". Later Captain unknown.
James B. Ezell, Co. "B".
Newt J. Vaughn, Co. "C".
Co. "D" may have been Frank Battle. Men from Rutherford County.
Robert H. Dudley (to major), Co. "E".
Robert Withers (to lieutenant colonel), Robert L. Morris, Co. "F".
Lemuel Oliver, Co. "G". Formerly Co. "B", Carter's Battalion of Scouts.
Norton, Co. "H".
Thomas Hollowell, Co. "I". Men from Rutherford County.
J. B. Van Houten, Co. "K".
Goodspeed's History of Tennessee says Captain E. I. Neil, from Marshall County commanded one company in Carter's Regiment. A microfilm copy of "Volunteers from Marshall County," in the Tennessee State Archives, gives a muster roll of Co. "II", 11th Tennessee Cavalry, commanded by Captain E. I. Neill. The names on the roll do not correspond with the names on other muster rolls of Co. "H", 11th Tennessee Cavalry, and it is believed that this company was in the 21st (Nathan W. Carter's) Regiment.
Captain Morris stated that Carter established a camp at Kittikaskia Springs, near Courtland, Alabama, and by midsummer of 1864 had a first rate battalion formed. On July 27, 1864 a Federal report of a skirmish on Big Name Creek, near Courtland, Alabama, spoke of "a Colonel Carter, with 200 men." When General Wheeler made his raid into Tennessee in August, Carter's command joined him just north of Lawrenceburg, and returned with his command across the Tennessee River, and again went into camp at Kittikaskia Springs.
On the 21st of September the regiment crossed the Tennessee River at Bambridge, and joined General N. B. Forrest at Florence. It was with General Forrest in the capture of Athens, Alabama on September 24, where it was in support of Rucker's Brigade, opposing the re-enforcements from Decatur, Alabama, seeking to come to the aid of the garrison at Athens. General Forrest, in his report said he ordered Colonel Nixon and Colonel Carter, each with about 150 men, to re-enforce Colonel Wilson, and the charge that followed resulted in the capture of the re-enforcing column. In this charge Colonel Carter was wounded. The regiment continued into Tennessee with Forrest, and when he recrossed the river October 8, again went into camp at Kittikaskia Springs.
By the time General Hood moved up for his invasion of Tennessee the regiment numbered 600 or 700 men. It was attached to Brigadier General W. H. Jackson's Cavalry Division with Lieutenant Colonel Withers in command, since Carter was still incapacitated by wounds. It took part in the Battle of Franklin, and then in the demonstration against Murfreesboro on December 5-7, where it served with Brigadier General William B. Bate's Infantry Division.
From Murfreesboro, the regiment moved to Shelbyville, where it was stationed until after the Battle of Nashville. According to Captain Morris, it was not until this time that the regimental organization was completed by the election of Lieutenant Colonel Withers and Major Dudley.
On December 22, General Hood instructed Lieutenant Colonel Withers to continue to gather stock, and to scout the country in the direction of Shelbyville and Fayetteville, and to report with his command to General Forrest when Forrest retired to the Tennessee River. Captain Morris said the regiment was cut off from both Hood and Forrest, and was ordered to disperse and rendezvous at luka, Mississippi. A Federal order dated January 6, 1865, at Edgefield, (now East Nashville) stated: "The object of the expedition is to pick up the many stragglers from the rebel army who are understood to be lurking in the country, particularly a regiment of Tennessee Cavalry under Lieutenant Colonel Withers, which is understood to be scattered in the counties of Davidson, Williamson, Wilson and Rutherford." Captain Morris reported that only a portion of the men made their way across the Tennessee River into Mississippi, but enough to form three companies.
On February 13, 1865, General Forrest ordered all the Tennesseans in his command to report to Brigadier General W. H. Jackson for consolidation into six regiments. As a result of this order, the regiment was merged into Nixon's Consolidated Regiment which was paroled at Gainesville, Alabama, May, 1865.