There were two different regiments of cavalry that were designated the 31st Texas Cavalry. Regimental Returns: Col.Peter Hardeman's Regiment Texas Cavalry, July 1863 and Col Peter Hardeman's 31st Texas Cavalry, March 1865.
Archives: "After the regiments of the Arizona Brigade were assigned to different Brigades, this regiment was known in the field as Hardeman's Cavalry and as 31st Regiment, Texas Cavalry. No official designation or recognition of Hrdeman's 31st Texas Cavalry regiment appears to have been given. Another regiment having been designated as 31st Cavalry"
Archive notes dated Feb. 23,1911: "In view of the fact that the numerical designation (31st) appears on only one record of the regiment, and no evidence is found that it was officially recognized as the 31st Regiment--there being also another Texas Regiment with the same numerical designation. It would be well to index it as Hardeman's Regiment, Texas Cavalry. The index has been amended accordingly"
It is assumed that 31st numerical designation of Hardeman's Regiment, Texas Cavalry was made without proper authority. It also appears, according to the records, that another regiment of cavalry had been properly designated as the 31st Regiment, Col. Trezevant C. Hawpe's Texas Cavalry.
My great grandfather Joshua D. Coffee was in Co. C, Col. Peter Hardeman's 1st Texas Cavalry Regiment, Arizona Brigade. Enlistment from July 8, 1863 until March, 1864. In Jan. 1865, he was transferred to the "Brush Battalion", Col. James G. Bourland's Frontier Regiment for the remainder of the war.