Made of wood. A screw inserted through the key(s) provides extra leverage.
This model is no longer available, but a similar model is available in plastic, though it is not as strong. Warning: the device makes the user stronger than would be expected. Keys may break easily in stiff locks or when using a copied key (copies are usually made of softer metal).
Swereco, swereco.com (plastic version) (last checked April 2020)
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This tip is from 2005