what is this

[ohioscott]

This brass fitting is located atop a hot water tank. has Watts brand with the faded description ?2 cfm@2"Hg
m

 

[Dumber than lumber]

That is something which lacks context.

 

[unslow1]

Water heater vacuum relief valve.

https://www.menards.com/main/plumbing/water-heaters/water-heater-parts-accessories/protech-marathon-water-heater-vacuum-relief-valve/sp623336/p-1444452250089.htm