I should probably include the image links.... The screw goes in the empty
threaded hole visible in the first image at the top of the 'tower'.
https://postimg.org/image/vbidm5d17/
https://postimg.org/image/ik47fmvjf/
(Note that I am not certain of the ID as an 870, as it is not as large as an 870
should be, but I can find no other models with the same fine adjust setup in any
catalog I have access to. If there is a better ID, I'd  appreciate it)

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