Help identifying old fitting

Hello everyone,

I'm wondering if someone could help me identify what this fitting is. The tub drain pipe
and overflow pipe join together via a T fitting and then into this fitting. Then the pipe exits
the fitting and presumably exits to the basement. The house is old so there is no P trap.

I currently have a back up into the tub and I'm wondering if this is a trap that can be cleaned
out. I ran some hot water down the tub and felt warm pipes up until this fitting, but not after.

Any information would be helpful. Thanks!

-Tony

It appears to be some sort of drum trap, but you do not show enough of the installation for us to determine how it is connected to the tub.