Well, I haven't had an opportunity to search for my files, but I did find a good amount of info on this topic by searching on the internet.

Note that Roland Dunbar calls the group at Selma a detachment, but the records indicate that four companies had been *reassigned* to Chalmer's Division.

Source 1:

1921 Tennessee Civil War
Veteran's Questionnaire
by
William Thomas McKnight
Enlisted in Co. A, transferred to Co. B

37. State in your own way your experience in the war from this time to the close. State where you went after the first battle-what you did, what other battles you engaged in, how long they lasted, what the results were; state how you lived in camp, how you were clothed, how you slept, what you had to eat, how you were exposed to cold, hunger and disease. If you were in hospital or in prison, state you experience here.

Were in battle near Rome Ga (horse shot from under me) lost mule in battle near Spanish Fort, Ala. Came near being captured (by yanks). Was in battle at Millnery Farm, Bluff Springs (near Pensacola Fla) shot by negro yank & I killed him. (Yanks fought us with negros). Yanks came out from Pensacola & cut RR at Pollard . With Forrest at West Point, Ponotoc & Tupelo. Was with Wheeler at Turkey Town & many other battles. Slept many nights in snows, sleet & saddle for pillow & hungry. Mobile & Blakely

Source 2:

"MILITARY HISTORY OF MISSISSIPPI", Roland Dunbar. Pages 424 - 427

This regiment was organized of Alabama and Mississippi unassigned companies after Sherman's raid to Meridian, February, 1864.

In March, 1864, in preparing a brigade of cavalry for the defense of Richmond, Va., it was suggested in the War Department that Colonel Armistead "bring seven companies out of the ten recently assigned to him and Lieutenant - Colonel Hamilton," and with the three companies in the Jeff Davis Legion, form a Mississippi Regiment for Gary's Brigade. But the brigade was otherwise formed.

May 1, Gen. S. D. Lee, at Demopolis, Ala., was notified that Gen. Gideon J, Pillow would report to him, with orders to complete a brigade, including Armistead's Regiment. Lee ordered the regiment to report to Pillow at Selma to be armed. Lee at this time moved Jackson's Division to the prairies around Columbus.

Pillow wrote from Oxford, Ala., June 10, "I have only the three small regiments, Armistead's, Ball's and Thomas'." The Twelfth Mississippi Cavalry, Colonel Armistead, was part of the command of General Pillow in his raid upon Sherman's communications June, 1864, Armistead commanding one of the cavalry brigades, including his regiment. Armistead drove in the Federal pickets at Lafayette, Ga., June 24, and took possession of the town, except that the courthouse, jail and hotel, which were held by the Federal troops, who resisted several attacks, in which the Twelfth Regiment lost 12 killed and 13 wounded, Colonel Armistead being dangerously wounded, and Major Redwood killed. During the struggle to capture the garrison a body of Federal cavalry sent out from Chattanooga arrived and made a sudden charge that stampeded the horses and compelled a hasty retreat. General Pillow returned with his command to Blue Mountain, Ala.

August 21, 1864, Armistead's Regiment, Lieut.-Col. Philip B. Spence commanding; Sept. 1, return of Armistead's Cavalry, Col. Charles P. Ball commanding, headquarters Talladega, Ala. Armistead's Regiment, present for duty 32 officers, 302 men; aggregate present and absent 750
Listed November 20, 1864, Sixteenth Confederate, Col. Charles G. Armistead, in Armistead's Brigade (otherwise Alabamian), in district of Central Alabama, Gen. Dan W. Adams.

In December, on receiving news of a Federal expedition from Pensacola to Pollard, Ala., Colonel Armistead moved his brigade to Bluff Springs, a march of 150 miles in fifty-four hours, and pursued the expedition on its return, December 16-17, General McKean, commanding at Pensacola, reported that "considerable severe fighting took place at all the streams from the Little Escambia to Pine Barren Creek." Colonel Robinson, commanding the expedition, was severely wounded, and in his troops 17 killed and 64 wounded. General Beauregard reported that our forces "acted with spirit and gallantry."

February 18, a detachment of the regiment was assigned to Armstrong's Brigade, in Mississippi, at the reorganization under General Chalmers. March 18, portion of Twelfth Regiment, reporting to Colonel Wade, transferred to Dillon's Second Regiment. Forrest's headquarters, Pickensville, Ala.

March 10, 1865, Sixteenth Confederate, Lieut.-Col. Spence. in Armistead's Brigade, near Mobile. Armistead took command of cavalry at Pollard during Federal advance from Pensacola, March 12.

Order of War Department, March 24, 1865, the ten companies of Mississippi Cavalry under the command of Col. C. G. Armistead will constitute the Twelfth Regiment, Mississippi Cavalry. The regiment, commanded by Lieut.-Col. Spence, participated in the defense of Spanish Fort, east of Mobile Bay, in March, 1865, and served as rear guard during the evacuation of Mobile and retreat to Demopolis. Major Cox, commanding officer of the regiment and the battle-flag, were captured April 15, about six miles from Tuskegee, Ala., by Private Shoef, Third Ohio Cavalry. April 28, Union officer reported that Colonel Spence's Cavalry was at the State line with pickets out to Citronelle.
Lieut.-Gen. Richard Taylor capitulated at Citronelle, May 4. 1865.

[Notice that this account has the capture of Major Cox occurring at Tuskeegee, rather than Talledega.]

Source 3: (origin unknown)

Assignments:

Armistead's Cavalry Brigade, District of Alabama, Dept. of Alabama, Mississippi, and East Louisiana (March - May 1865)

Armstrong's Brigade, Chalmer's Division, Forrest's Cavalry Corps, Dept. of Alabama, Mississippi, and East Louisiana [detachment] (February-May 1865)

Source 4: (origin unknown)

One researcher states that “it is known that William Hardy of Company I fought in this skirmish”, in reference to Wistler (or Eight-Mile Creek Bridge) April 2-9, 1865.

Source 5:

The 62nd Ala. Inf. Regt. was also at Wistler, according to “The Confederate War Records for the 40th Alabama and the 62nd Alabama; The military units of Hiram Steele and Elijah Fincher; 1861-1865”.