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

Attachment 15617

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.