I incorrectly assumed that you had the letter in your possession. Sorry, didn't mean to preach.
I have examined the 1919 pension application for 84 year-old "J. C. Bishop" of Floyd County, Virginia. Unfortunately this 10 page microfilm is 80% illegible. However, it appears that Bishop joined the 45th Virginia Infantry in September 1864. [He was probably conscripted.] His name does not appear in the service records of the 45th. Records show that he was transferred to the 17th Virginia Infantry. The 17th Virginia Infantry was one of 5 regiments in General Montgomery Corse's Brigade. This brigade was one of 4 brigades in General George E. Pickett's Division, which was part of General James Longstreet's First Army Corps in the Army of Northern Virginia. Perhaps someone else can comment on Bishop's service with the 17th Virginia Infantry. I do not own that volume of the Virginia Regimental Histories Series.
Why was Joseph Bishop transferred from the 45th? Why was he at Castle Thunder? Can't say for sure, but the Civil War and the Confederate cause was unpopular among a large portion of citizens in Floyd County, VA.
J. C. Bishop may have been seen as a "bad influence" on the the men in the 45th Virginia, most of whom were from the western counties of Carroll, Bland, Wythe,Grayson, and Tazewell. A little research shows that Bishop's brother Henry Wesley Bishop was a member of the 54th Virginia Infantry. This Bishop went AWOL from the 54th in May 1862, refused to reenlist, and was conscripted [forced] to rejoin the unit. Henry W. Bishop was captured at Missionary Ridge, Tennessee in November 1863. He joined the United States Navy in January 1864.
Joseph C. Bishop's widow, Luticia Bishop filed a pension application in 1920. Regrettably this 4 page application is difficult to read as well. See:
I am not sure about the Virginia State Library policy regarding the viewing of original documents [as opposed to microfilms]. You might inquire at: Archival and Records Management -- (804)-692-3600