I saw something new to me today. I was removing a metal 4/0 box from a ceiling, it was installed years ago as old work. It was an ordinary 4/0 box, but it was held into place by two flat pieces of metal, 4.5" or so long, that sat on top of the ceiling. Then there were two half moon shaped pieces that fit flush to the bottom of the ceiling, with machine screws that thread into the top two pieces. Then there were two more holes that line up with the threaded ears on the 4/0 box, which had screws holding the 4/0 box to all this. Finally, there were two more threaded holes in the half moons to hang the fixture from.

The only markings said "patent applied for"

I would like to find some more of these if they are still made. I do a lot of old work and sometimes it helps to have options in unusual situations, like when the hole in the ceiling is too big for a normal cut in box etc.