What is this regulator part

[MountainManJoe]

I was washing this CO2 regulator and brass piece fell on the floor. I can't figure out where it goes. Any ideas?

 

[day_trippr]

You would use that insert to retain the original square O-ring used on that regulator stem.
It's not used with the much thicker nylon gasket, which is fine...

Cheers!