I guess that's the green spring. Looks like Midway is out of stock.

What's interesting to me is the bar it holds is referred to as "Barrel stop", which I believe is the correct name and purpose.

It's usually referred to here as a "Take Down Lever/Bar".

It's interesting that it popped out. It looks like it goes down under some structure. I've never had mine out but did restrain it when I put in rhe NDZ barrel stop.

I wish you quick success in finding one.