Wierd Drum Trap

My daughter lives in an old Chicago building and has a stopped up bath tub drain. It appears to have a drum trap. I have opened the trap and find the inlet appears to be on the bottom side wall of the trap. The only other opening I can find goes out the bottom center of the trap. Is it a trap? How does it work? Water moves from the tub to the trap but does not exit. What to do?????

Thanks

Answer

I found the answer. Despite the pipe directions and my assumptions, when I sponged the drum empty and then ran a little water in the tub, I found the hole in the bottom of the drum was the inlet and the side opening was the outlet. From that, it was a simple matter to snake the proper outlet pipe and find the hairball clogging the drain. Everything is fine now and the tub works better than it has since she bought the place eleven years ago.