The plaques went missing years ago, presumably as a prank.
Recently Marisa LaFalce, Cornell Chimes program coordinator, received a package containing the missing plaques. They had been sent in by an Andrew D. White of Florida, who wrote:
The enclosed were liberated about half a century ago at a time when the tower was closed to visitors and the old works were no longer on display. The entrance to the tower was provided by a chimemaster, and the inspiration, and the screwdriver, by a professor of engineering.
They are returned to you in the hope that they can find an appropriate home.
If you know the story behind the missing plaques, please email The Essentials.