The house is old enough that it probably had an outhouse at some point. Most of the houses on the street started early enough to be before the miracle of flushing.
The lighting of "pocket bathroom" was done with a flashlight during a time exposure that took as long as the lighting required. This is often my strategy if I can shoot in complete darkness.
There is a photograph of a section of the same room in the lower right corner of the window, making a picture within the picture.