Hello Tom, you brought out some valid points. At Camp Chase as an example there was at Union deserter from Kentucky who elected to join the Confederates rather than to wait for exchange. He was subsequently captured by other Union regiments and taken to Camp Chase. The Confederates had no problem in telling Federal authorities who he was and a court martial was formed but he died prior to the court marital and is now buried as a Confederate.
One thing we should take into account with these particular former Confederates and Union deserters posing as Confederates that were taken prisoners at Gettysburg is that they were paroled because of their desirous intentions of joining the Union Army very quickly and thus separated from the general prison population, thus helping keep their secrets secure. During the 1880's a former Confederate who had been galvanized even gave an affidavit supporting a Union deserter in getting his Federal pension.
I believe I know how one Union deserter changed into a civilian clothing but don't want to let the cat of the bag quite yet.
I agree with you insofar as Confederate prisoners having no problem in turning in a Yankee deserter. But we should make note that we are talking about deserters in general. Very few hard line Confederates who were captured at Gettysburg would have elected to join the Union Army. While it's highly subjective let's explore a scenario. A small group of Confederate deserters take shelter in an abandoned barn. A man walks in with the same idea dressed in civilian clothing. Both the man and the group of Confederate deserters are on edge. Soon the man admits to being a Union deserter and the other Confederates admit they too are deserters. Now a bond starts to emerge misery loves company type of thing. They are all in the same boat. One Confederate deserter says I have a brother in a Confederate unit at Vicksburg, just say your with that unit. While it's true the Union deserters could have come up with the numerical regimental unit at random my gut feeling is they had some inside help. A large amount of Confederate deserters at Gettysburg were born in northern states thus again having something in common with some of the Union deserters.
Your point about the Union knowing what units were from what areas is a valid point. I could not believe how many former Confederates captured at Gettysburg gave their units as being in the Western Theatre. My first thoughts were "don't you Federal authorities know this unit was more than 500 miles away?" My only explanation to these strange statements are that things were happening fast and in volume in the days right after Gettysburg. The ANV was getting away and the provost marshal's were working overtime. I don't believe the average junior Union officer knew all the Confederate units at the Gettysburg Campaign. When the Confederate prisoners in general were taken to northern prisons the atmosphere was not much better insofar as getting valid information. The Union clerks were working night and day for awhile and many times just took information originally supplied by the Confederates to the Union soldiers. In a perfect world the arresting Union officer should have reported to the Provost Marshal where and when the Confederate prisoners were apprehended after turning them over. The Provost Marshal would then try and extract additional information and the information would be forwarded. However the emergency of 1863 was anything but perfect. It appears the primarily concern was getting the Confederates to prison as quickly as possible.