You have both types of contacts on the one unit, so you will always have the correct one!
Normally Open (N.O.) and Normally Closed (N.C.) per pair.
Contacts are closed cicuit when the magnet is alongside the switch. For alarm installation this is denoted as the "normal" state as the door/window is secure and the alarm is armed.
||Dimensions identical on switch and magnet
|Overall Screw Spacing (H)
|Attachment / Securing Method
||Screw Down,Self Adhesive Tape/Pad