What is this on the copper drain line

[mathew stouffer]

Town home built in 84. Copper drain line, I was going to steal it:) What is this.

[Jerry Peck]

That's a Sovent aerator ... that is a one pipe system where the vent and waste are in one pipe.
http://www.sovent.com/downloads/SE%20SOVENT%20DESIGN%20MANUAL%20101o%20new011007.p df

[mathew stouffer]

Thank you. First time laying eyes on one of those.