FL5 rear motor mount question

[HKSpeed]

Why does the stock FL5 rear motor mount have a slot in the bolt hole? Aftermarket mounts don't have it. Neither does the FK8 stock mount.
Photo from unity performance of 11th gen stock mounts (left is not-Type R, right is Type R)

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[VtecSlayer]

Probably just the tooling used at the factory to insert the larger piece.