There are two different types of locking knobs. The rhomboid locking knob is attached to the locking knob with two screws; the arm length is 9 cm.
The flat locking knob is attached to the locking knob with one screw; arm length is 9 cm.
The locking knob is an important complement to electric door openers and is important to have in the event of a power outage.
The user chose to position the safety lock at the bottom rather than at the top.
For example Swereco, www.swereco.se (last checked May 2017)
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This tip is from 2005