My house was built in 1928 and I found one of my ceiling lights was "mounted" to this (it was actually screwed into the ceiling; hope they didn't nick a wire).
I'm trying to figure out what this is, mostly so I know how is it attached to the ceiling joist, how the electrical (possibly very old BX) may or may not be attached, etc. My goal is to put up a new fixture, though I'm currently thinking i won't touch the old wiring.
I finally figured this out, more or less. See two pictures below and my brief explanation.
First, I decided to trim away some of the plaster and verified the wiring is in an older box and is indeed BX.
The picture below is with that metal cap thing removed (the middle part with the screw has threads on the outside). I ended up taking out that screw and replaced it with a slightly longer screw so I could attach a crossbar.
This is what I removed; I'm not going to speculate how it was used, but I assume there was originally a hanging fixture.
