I found the following information that may be about D.S. Howell and Smith's Regiment, T.S.T.

According to the NPS CWSSS, there was Col. George Smith, 2nd Cavalry, Texas State Troops, 6-mo. (1863-1864. I don't know where the regiment was raised but there was a Pvt. T.S. Howell in the regiment. "T.S." could be D.S. Howell.

There was Geroge Washington Smith (1796-1876) who was born in Wilson Co. Tennessee. In 1814, Smith served under Brig. Gen. John Coffee at the battle of New Orleans. In 1846, he served under Col. John Coffee Hays, 1st Texas Mounted Riflemen at the battle of Mexico City. After the Mexican War, Smith settled at his home in newly formed Fannin County, which later became Collin County. Smith died in Collin County and is buried there. This George W. Smith, at the age of 67, could be the gentleman who raised the 2nd Texas Cavalry, T.S.T. in 1863-1864.