[Polybius]

So if I come across a catch of letters written by Abe Lincoln when he was in the Illinois House of representatives I would have to give them to the State of Illinois rather than be able to sell them or even receive a tax credit for the donation.

If they were personal correspondence they belong to you.

If they were official correspondence, they belong to the Government.

[Steve_Seattle]

"If they were official correspondence, they belong to the Government."

Unless the government sold them, auctioned them, or officially disposed of them, which they do with millions of documents.