If I'm not mistaken, I think that's where the leveling screw fits. In OE radiator, you install a different leveling screw (I think that's what it's called) to adopt radiator to either an auto or manual trans.
I don't know if that part is able to be found separately. I'll look at realoem and come back with what I see on my car...you can confirm on yours.
I suspect, though, that a place like one our AM sponsors might carry that as a separate part. If not, new Behr radiator for $140 or so would be the worst...for the radiator...and then, to make it all nice and reliable for the next four years, you could spend another $300 in parts and be done with worrying about it ...except for flush every 2 years.
Back in a second...
Doesn't look like that's a 'bmw' part, but check the diagram, as I'm not familiar with auto trans, so maybe it's not what I think, but part of the bracket ...or something.
Try Bavauto, Turner, Pelican, ECS...or once you confirm the part, maybe you could ask if someone has it around.
Actually, if yours happens to be a manual (didn't think so), but I actually might have that screw...find picture and confirm stuff.
Of course, when you unscrew that you'll have drained your coolant so will need more...did I see green coolant?
Oops...whole other lecture on that...you can get it from searching (coolant...in title search).
And...distilled!
You don't seem like you own a beagle, is that right?