I copied this from the FAQ section of the 1981-present forum. You can go there to read the entire post.

If a handgun in the recalled range has already been returned to the factory for testing, a red letter “C” that is circled will be stamped on the box label, indicating that the gun passed re-inspection. No marks are made on the cylinder of checked guns, only the red “C” in the circle stamped on the box label.