Bottom bracket removal, what is this part? I have a new (old) bicycle, and I am in the process of disassembling it.

I arrived to the bottom bracket (with a cottered crank), and unscrewed the ring without problems, but then I'm faced with a part that I don't think I've seen before (circled in yellow in the picture below, the part next to the frame with the threads and a strange "groove").



I've disassembled several BB, but this is new to me (normally I have a nut and unscrew it, I bet this may be another kind of nut), and I don't know if this can be removed and how?

Is this something that I can unscrew, and does anyone know of the technique/tools to do it?

(By the way, the other side has the same part, but as far as I can see there is no thread (visible at least))
It's a bottom bracket cup. There are specialty tools you can use to remove it such as the HCW-11 on this page, or if it isn't rusted in, you can probably accomplish the same with an adjustable crescent wrench.  Under there are some ball bearings, so make sure you don't lose any when you open it up.  The bearings should be in a cage, which would stop you from losing them, but with an older and worn bike such as the one shown in the picture, the bearing cage may be broken.