Blocker bolt is used with Andersen Frenchwood Outswing door, passive lock mechanism conversion.
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The blocker moves down when the Passive door is Locked, allowing the hooks from the active door to engage. This prevents the locking of the Active door, unless the Passive door is locked first.

The Blocker bolt is needed only when the passive lock does not have a pin installed.
If Purchasing Passive Kit it has the Pin attached and the cap is included with kit.