Hey. I would try looking up an unboxing video for your model of monitor and see if the video shows you how it came out of the box in the first place.
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crisp flicker
Got it, I am not familiar with that style of mount, so if you did buy the monitor from a store directly, then that would be my next recommendation. Bring it all in, say all the original hardware is there, but could not get this piece back in the box.