These are powerful neodymium magnets housed inside of a steel cup. The cup protects the magnet and also increases the magnets power by redirecting one polarity towards the other side. The Magnet embedded in this cup has a countersunk hole that goes thru the entire assembly. The countersink feature allows the magnet to be mounted to any surface with a standard 82deg flathead screw.
The maximum pull strength is measured by pulling the magnet directly off of a piece of steel.