That's an excellent question (which I don't know the answer to unfortunately). But people sometimes forget that every object we have today had to travel through time from the past to the present. Sometimes it's amazing to ask questions and discover strange things. Like that enginehouse building they have at Harper'sFerry where John Brown was captured--it spent some time in Chicago. It didn't sit in Harper's Ferry the whole time.
I noticed a barn on some historic farm had siding put on with modern nails and asked about it . The docent insisted everything was just the way it had always been. I don't remember if I finally got an honest answer or not, but it was obvious to me that the log barn had had modern sawn wood siding put on it with wire nails, probably while it was still being used and that was a practical thing to do.
And it's even worse for small artifacts. They get painted, repainted, "restored" to first be put on display, then restored more "correctly" later. Often nobody realizes they're rare and valuable until it's too late: the last private's sack coat or dog tent or the last type of ambulance that was used up in the post-war army, then sold to some farmer, then recognized in some barn and "restored" a dozen times. And everyone wants to claim it's just the way it was, no changes, perfect restoration.
I hope someone has the answer. Lee may be lucky in that people realized immediately that the items had historic value, and so surely preserved and cared for them right from the start.