Both come from a recently purchased kit and I don't need them.
The 18453 is brand new without a single scratch on it. It's a 28.5mm extension ring most commonly used with the Nikon 105mm and Tokina 10-17 with 1.4x teleconverter, 12-24mm, etc. No idea how they'd be used on Canon rigs so take a look at the charts for more info.
The 18457 extension is used and is in good functional shape. It looks like the previous owner used it with the Nikon 17-35mm, not sure of other uses.