The only "Sgt Yerby" I can find is

Joseph T. Yerby
Regiment Name 55th Virginia Infantry
Side Confederate
Company L -
Soldier's Rank_In 1st Sergeant
Soldier's Rank_Out 1st Sergeant
Film Number M382 roll 62

Co L = Capt. Charles N. Lawson's Co; organized March 5 or 6, 1862, and was composed largely of members of the 92nd Regt Va Militia; enlisted CS service March 25, 1862, for three years or the war;

U.S. Civil War Soldiers, 1861-1865
Name: Joseph T. Yerby
Side: Confederate
Regiment State/Origin: Virginia
Regiment Name: 55th Virginia Infantry
Regiment Name Expanded: 55th Regiment, Virginia Infantry
Company: L
Rank In: First Sergeant
Rank In Expanded: First Sergeant
Rank Out: First Sergeant
Rank Out Expanded: First Sergeant
Film Number: M382 roll 62

55th Regiment, Virginia Infantry
55th Infantry Regiment was organized in September, 1861, by the addition of four companies to W.N. Ward's "Essex and Middlesex" Battalion of Virginia Volunteers. It was assigned to General Field's, Heth's, H.H. Walker's, and Barton's Brigade, Army of Northern Virginia. The 55th participated in the campaigns of the army from the Seven Days' Battles to Cold Harbor, then was involved in the Petersburg siege south of the James River and the Appomattox operations. It sustained 108 casualties during the Seven Days' Battles, 32 at Second Manassas, 11 at Fredericksburg, and 110 at Chancellorsville. Of the 268 engaged at Gettysburg, twelve percent were killed, wounded, or missing. In February, 1865, the 47th and 55th Regiments were consolidated, but only 2 officers and 21 men of the 55th were present at the surrender. Its field officers were Colonels William S. Christian and Francis Mallory; Lieutenant Colonels Robert H. Archer and Evan Rice; and Majors Thomas M. Burke, Robert B. Fauntleroy, Charles N. Lawson, Andrew D. Saunders, and William N. Ward.

I don't know if this is the same one but

Joseph T. Yerby
Regiment Name 40th Virginia Infantry
Side Confederate
Company H - Lancaster Grays
Soldier's Rank_In Private
Soldier's Rank_Out Private
Film Number M382 roll 62

Co H = Lancaster Grays; Lancaster and Northumberland Counties; organized Dececember 18, 1859; formerly called Co C; first called Co H on rolls as of December 31, 1862 to August 31, 1863; Enlisted May 22, 1861, for one year from April 25, 1861; reorganized April 23, 1862.

U.S. Civil War Soldiers, 1861-1865
Name: Joseph T. Yerby
Side: Confederate
Regiment State/Origin: Virginia
Regiment Name: 40 Virginia Infantry
Regiment Name Expanded: 40th Regiment, Virginia Infantry
Company: H
Rank In: Private
Rank In Expanded: Private
Rank Out: Private
Rank Out Expanded: Private
Film Number: M382 roll 62

40th Regiment, Virginia Infantry
40th Infantry Regiment completed its organization in May, 1861. Its members were recruited in Northumberland, Richmond, and Lancaster counties. After serving in the Aquia District, the unit was assigned to General Field's, Heth's, and H.H. Walker's Brigade, Army of Northern Virginia. It participated in the campaigns of the army from the Seven Days' Battles to Cold Harbor, then was involved in the Petersburg siege north of the James River and the Appomattox Campaign. It sustained 180 casualties during the Seven Days' Battles which was about half its effective force. The unit lost 4 wounded at Cedar Mountain, had 14 killed and 73 wounded at Chancellorsville, and of the 253 engaged at Gettysburg more than twenty percent were disabled. Many were captured at Sayler's Creek and only 7 men were included in the surrender on April 9, 1865. Field officers were Colonel John M. Brockenbrough; Lieutenant Colonels Fleet W. Cox, Arthur S. Cunningham, and Henry H. Walker; and Majors Edward T. Stakes and William T. Taliaferro.

and

Joseph T. Yerby
Regiment Name 9th Virginia Cavalry
Side Confederate
Company D - Lancaster Cavalry
Soldier's Rank_In Private
Soldier's Rank_Out Private
Film Number M382 roll 62

Co D = Lancaster Cavalry, Lancaster County; 0mustered into service June 16, 1861, for one year from April 25, 1861 (date of enrollment); assigned 1st Bn Va Cav.

U.S. Civil War Soldiers, 1861-1865
Name: Joseph T. Yerby
Side: Confederate
Regiment State/Origin: Virginia
Regiment Name: 9 Virginia Cavalry
Regiment Name Expanded: 9th Regiment, Virginia Cavalry (Johnson's)
Company: D
Rank In: Private
Rank In Expanded: Private
Rank Out: Private
Rank Out Expanded: Private
Film Number: M382 roll 62

9th Regiment, Virginia Cavalry (Johnson's)
9th Cavalry Regiment was formed in January, 1862, using the 1st Battalion Virginia Cavalry as its nucleus. Its companies were from the counties of Stafford, Caroline, Westmoreland, Lancaster, Essex, Spotsylvania, Lunenburg, King William, King George, and Richmond. The unit served in W.H.F. Lee's, Chambliss', and Beale's Brigade, Army of Northern Virginia. It fought in the Seven Days' Battles and the conflicts at Gainesville, Second Manassas, Sharpsburg, Fredericksburg, Dumfries, Rapidan Station, Brandy Station, Upperville, Hanover, Gettysburg, Williamsport, Funkstown, and Culpeper Court House. The 9th went on to fight at Bristoe, Mine Run, The Wilderness, and Todd's Tavern. Later it skirmished around Richmond and Petersburg, then was active in the Appomattox operations. This unit reported 32 casualties at Upperville, lost four percent of the 490 engaged at Gettysburg, and had 22 disabled at Williamsport. It surrendered 1 officer and 26 men. The field officers were Colonels Richard L.T. Beale, John E. Johnson, William H.F. Lee, and Thomas Waller; Lieutenant Colonel Meriwether Lewis; and Major Samuel A. Swann.

U.S. Civil War Soldiers, 1861-1865
Name: Joseph T. Yerby
Side: Confederate
Regiment State/Origin: Virginia
Regiment Name: 92nd Virginia Militia [Lancaster County]
Regiment Name Expanded: 92nd Regiment, Virginia Militia
Company: A
Rank In: Private
Rank In Expanded: Private
Rank Out: Private
Rank Out Expanded: Private
Film Number: M382 roll 62

VA Militia: 4th Division, 14th Brigade, 92nd Regt[Lancaster County] - Col Samuel Downing, 1861
Regimetn called into service by proclamation of the Governor, July 13, 1861, to rendezvous at Lancaster Court House. Rolls show Co A enlisted May 22, 1861 for one year. Some members of this regiment sugsequently enlised in Co L, 55th Regt Va Inf.
Co A = Capt Isaac Currell's Co; enlisted May 22, 1861 for one year

1880 United States Federal Census
Name: Joseph T. Yerby
Home in 1880: White Stone, Lancaster, Virginia
Age: 52
Estimated birth year: abt 1828
Birthplace: Virginia
Relation to head-of-household: Self (Head)
Spouse's name: Isabella F.
Father's birthplace: Virginia
Mother's birthplace: Virginia
Neighbors: View others on page
Occupation: Farm
Marital Status: Married
Race: White
Gender: Male
Household Members: Name Age
Joseph T. Yerby 52
Isabella F. Yerby 41
Marry P. Yerby 22
Thomas A. Yerby 20
Llewellyn Yerby 18
Joseph T. Yerby 15
Jane F. Yerby 14
Winfield I. Yerby 11
Howard L. Yerby 5

1910 United States Federal Census
Name: Joseph T Yerby Sr.
Age in 1910: 82
Estimated birth year: abt 1828
Birthplace: Virginia
Relation to Head of House: Father
Father's Birth Place: Virginia
Mother's Birth Place: Virginia
Home in 1910: White Stone, Lancaster, Virginia
Marital Status: Widowed
Race: White
Gender: Male
Neighbors: View others on page
Household Members: Name Age
Joseph J Yerby Jr. 45
Alice M Yerby 48
Joseph T Yerby Sr. 82

Federal Slave Schedule of 1850 & 1860 indicate he was owner of 4 or 5 slaves.

His compiled military records should tell you.