I have an existing electrical box that appears to support ceiling fan installation but it is missing the 2 female screw holes I'm used to finding in these boxes.
Is there some sort of intermediate piece that attaches to the two existing post screws? I cannot find any brand or model information in the interior.
Based on those bolts, I would assume this is a retrofit ceiling fan brace box like this one, but I have no idea where the screw ears would have gone. Even boxes that aren't meant to have a fixture or fan hanging from them should have screw ears to attach a cover to the box.
I would just pull that one out and replace it with a new ceiling fan brace kit. They are cheap enough that trying to find parts for yours doesn't make much sense.
Of course maybe this is an odd style that I haven't seen before...
This looks like a Legrand 22200 retrofit box. They typically have a riveted screw on each side, but I don't see either.
You might not need to buy a whole new brace, however. Take it down and go to the hardware store. They should have a box that will work with it as-is.
They make cross braces for dealing with things like this. It's a bar of metal with threaded holes at the appropriate places, then two plain holes for going into those studs.
I gather the studs are short of the edge of the box. To attach to those, screw a coupler onto the studs, this being a length of hex stock threaded on the inside.
I have never seen a box like this before and have to assume it's a manufacturing defect or a ticked off contractor. If you have access to an attic or crawl space above this, you can just remove it and the support bracket and replace it with a different one. Otherwise remove the "u" bolt nuts, remove the box and look for a replacement similar to the one shown below.
The 10-32 screws support the fan. The fan itself usually has the bracket and cover that matches, I have used them on new construction.
