A "check protector" refers to anti-alteration measures used to prevent someone from unscrupulously modifying a check to appear to be an amount higher than what the check writer originally intended.
Several examples and methods are shown here:http://www.officemuseum.com/check_protectors.htm
I've sen them fairly frequently on checks from the Battleship era (1898-1901), but never from the Civil War period... and the website above indicates that they first made their appearance in the early 1870s.
I stumbled across the check below at online auction and what caught my eye was not the stamp or document, but rather the check protector running across the face of the stamp, and the fact that it was from 1865.
I suppose that theoretically, the protector could have been applied long after the fact, possibly by the company using old checks as test documents for later protectors (the company in question was still in business up until 1990), however there are two aspects of the protector that make me think it was contemporarily applied:
1. The dollar amount of the protector does cover the amount that the check was written for.
2. The ink color of the protector is almost an exact match for the ink used in printing the check. The odds of it being that close of a match if applied decades after the fact are slim.
Below the document is a closeup image of just the protector.
UPDATE: It turns out it WAS applied long after the fact. I completely missed that this isn't a check, just a receipt. There would have been no reason to use a protector on this document. So file under interesting oddity, nothing more. Well, at least the cost was nominal.