Mike,
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Yes, I think you are correct about the military unit in which my "George Washington Watson (1845-1920)" initially served, even though he said he was age 20 at the Apr 1861 enlistment into the Texas Mounted Rifles when he was really age 16. Since he lived in Rusk Co TX for the 1860 and 1870 Rusk Co TX censuses, it certainly makes sense that he would sign up in Marshall, Harrison Co TX. Those who signed up before May 1861 were either gung-ho Southern sympathizers or adventuresome kids.
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This G.W. Watson is listed on my web page as one of the "Additional Soldiers in Bourland's Regiment" that were not listed as a member of Bourland's Regt in the archival records.
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.http://www.bourlandcivilwar.com/AdditionalSoldiers.htm
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Watson, George Washington Watson (1845 Marion County TN-1920 Kiowa Co OK, Mountain Park Cem); m-1869 Peatown, Rusk Co TX to Tempie Ann English, 10 ch; found in censuses: 1860 and 1870 Rusk Co TX; 1880 and 1900 Montague Co TX; and 1910 Kiowa Co OK. "While his company was on 'leave' George returned too late to join his company on their departure back (missed his ride), so he was sent to the North and West Frontier to serve in Bourland's Regt. .. [After the Dec 1, 1864 muster]. ... Family tradition states: ..Their [men of Bourland's Regt] duties were to guard the frontier against the Indians. ..They did much scouting. George was with a detachment of Bourland’s men on one occasion, snowbound in the Wichita Mountains, and their horses had nothing to eat for several days except cottonwood tree bark [ca. Feb 7, 1865]." ..sent by Coy Wilbur Watson of Lawton OK. (see vIp300)
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Records show "Pvt. G.W. Watson, age 20, enlisted Apr 19, 1861 at Marshall, Harrison Co TX; mustered May 23, 1861 at San Antonio TX into Capt. Samuel J. Richardson's Co, 2d Regt, Texas Mounted Rifles (later called Texas State Troops). Assigned to Fort Inge, Uvalde Co TX; Fort Clark, Kinney Co TX; Marshall TX; and Camp Ford (prison for Union soldiers), Smith Co TX. This company at some time became an unattached company. In August 1863 they were relieved as guards at Camp Ford by Border's-Anderson's TX Cav. Regt. They then went to Arkansas. Richardson was supposed to have joined up with Walter Paye Lane after he left Camp Ford. The last record for Pvt. Watson show him, “with leave” starting May 1, 1864, no camp location shown. per Mike Thompson.
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The following entries are from my 146-page Name Index.
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Border
John Pelham (1819 England-1873 Iberia Parish, LA); m-1844 to C.E. Harding, 6 ch, who m-1887 Gov. O.M. Roberts; arr 1835 in TX; lived San Augustine Co TX; Border's Cav. Battn. became Apr 27, 1864 Thos. Scott Anderson's Cav. Regt; reorganzied on Feb 27, 1865 as Border's-Anderson's Cav. (Col, Border's TX Cav, CSA), 177

Anderson
Thomas Scott Anderson, absorbed both Lt-Col Phil Fulcrod's Cadet TX Cav Battn and John Pedham Border’s CSA Cav. Regt. of Angelina Co TX on Apr 27, 1864; reorganized Feb 3, 1865 to become Border's-Anderson's (Col, Border's Regt-Anderson's TX Cav, CSA)

Lane
Walter Paye Lane (1817 Ireland-1892 Harrison Co TX) single; arr 1836 TX; promoted Mar 17, 1865 to Brig-Gen (Capt, Co A, 3d TX Cav; Lt-Col, 1st Lane's TX Partisan Rangers, CSA), 118, 189, A-5, A-6, A-7, A-8, A-34, A-37, A-344, A-345, A-346
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Question: I could not find Samuel J. Richardson nor Thomas Scott Anderson in the census records. Did you find them in the Texas censuses?
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Patti, prochette@Juno.com
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Quoting Mike:
Patti,
Not sure this is your man.
Pvt. G.W. Watson, age 20. Capt. Samual J. Richardson's Co 2d Regmt. Texas Mounted Rifles. Enlisted April 19, 1861 at Marshall, Texas. Mustered in May 23, 1861 at San Antonio,Texas. Assigned to Ft Inge, Ft Clark, then Marshall, Tx and Camp Ford, Tx.
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This company at some time became an unattached company. In August 1863 they were relieved as guards at Camp Ford by Anderson/Border's Regiment Tex. Cav. They then went to Arkansas. (From my research of Anderson/Border's Regmt. Richardson was suppose to join up with Walter P. Lane after he left Camp Ford). The last card on Pvt. Watson show him, with leave starting 1 May 1864. No camp location shown.
Mike