The lock strip allows you to fix the keyboard halves to each other, resulting in a one-piece UHK. In order to install it, first remove four keycaps as shown below.
Next up, remove the two scews at the end of the strip with a PH0 screwdriver, place the strip to its shown position, and fixate it with the screws. Please screw properly and lightly.
That's it, you're good to go!