The locking mechanism is integrated into the cover of the stamp. The cover has a rectangular button integrated. You may lock the stamp in two positions. The lower position is to replace the ink pad or to turn the print wheels. The upper position is to release the pressure from the inkpad while the stamp stands idle e.g. for the night.
Some REINER models feature a metal locking plate sitting inside the outer frame, or some older designs have a locking pin placed at the spring tube below the handle.