I am helping a friend replace some lights with ceilings fans, and all was good until I ran into this funny looking box. Most of the lights were installed with shallow fan rated boxes bolted to the joists, so the swap was easy. This one, however, has some sort of brace between the joists. There is a sliding metal piece on the brace, that the box bolts to. I just wanted to know if this is a ceiling fan brace. I hope it is, as I don't feel like crawling around in the attic (which due to position I'd have to if I want to replace this one. I have attached some pics. Any info would be greatly appreciated!