Hello George, Looking at fold3 I don't see J. H. Fuller as in Company D of the 3rd Tennessee Infantry (Clack's) I did not look under miscellaneous however. While I did see two surnames as Fuller I did not see his. I further realize you did not use the reference as fold3 in your citing.

As you know the 3rd Tennessee Infantry (Clack's) was not taken as POW's at Vicksburg, MS. on July 4, 1863 as a unit but they were in battles in and around the Vicksburg Campaign. It is entirely plausible he was taken prisoner at Raymond or Jackson but I would not think a date of July 4, 1863 would have been used as this date was primary used for those Confederate troops that were trapped in the siege of Vicksburg in which Clack's unit was not a part of. It's also possible he may have been a stray who might have ended up within the siege but I'm not seeing any reference to that.

Another thing that bothers me is that I know of no Confederates who turned galvanized at the surrender of Vicksburg but this too may have occurred on some isolated circumstances that I'm not aware of. I'm sure there were a few Confederates at Vicksburg who probably did not want to be paroled with their Confederate units.

The 3rd Tennessee Mounted Infantry was not known for having galvanized Yankees such as the 3th North Carolina Mounted Infantry as an example. But I also note there were no hard and fast rules that a former Confederate could not enlist in any Union unit. I saw what I thought may have been the soldier in question on the 1860 United States census as John T. (Tuck) Fuller living in Blount County, TN.