Mike,
That white receptacle is one piece. It would mount to the wall and your Knob and Tube wiring would attach to the brass screws. I haven't seen many of these installed, but I have only found the one. I have seen many,many of those old black boxes with the 2 holes for the loom. I have a picture of the loom which I will send in. As for the Edison based sockets that you mentioned, I haven't seen them either. I worked in northern NJ and many towns/cities have buildings 100-200 years old. Many still have the old wiring still in place and in use today.
Caper
