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There is a plate behind my thumb latch

If your lock has an additional plate under your thumb latch, you will need to remove it. You’ll also need to remove the thumb latch. Use these screws to attach the August mounting plate to your door.

Use the back plate to determine which mounting plate is best for your lock. Put the screws through the back plate, then select each one of the mounting plates and slide it up the screws.  When the screws fit straight, you'll know you're using the correct mounting plate.