I recently bought this A1267 display from the recycled goods store, and I have absolutely no idea what this thing is on the stand.
My best guess is a mount, but it doesn't look like it would fit on my VESA mount. I tried to search up A1267 mount images but found nothing that matches this.
Didn't occur to me that it might be holding the monitor stand together. It definitely isn't first party, but it seems very well made so I'm guessing a quality third party thing. The steel is a good 4-5mm thick.
I'd be curious to take it off except now you've mentioned it might be holding it together I'm a little uncertain on that haha
Otherwise it seems like my question is mostly answered based on the lack of responses - it's not a common aftermarket thing and definitely wasn't an official Apple accessory.