The slots should fit the over the bolts with washers and the screw holes are for mounting the light to that ring.

 

You have an old-timey fan mounting box. Maybe not all that old, but it predates today's convenient fan/fixture combination boxes.

Normally you could easily remove it and install a 3 1/2" fixture box in its place. But someone really wanted it to be exactly where it is and notched out the joist. As a result it's impossible to tell what is supporting the box. Can you remove it and take another photo? Use a sharp utility knife to remove the patched drywall that is blocking the box, then just unscrew the box and pull it down.

A fan removed from a box like this may have an oil filled motor. Don't tip or invert the old fan until you know for sure.

 

Thanks, Nealtw. I thought about that originally but unfortunately the slots are also 1/8" so the 3/16" bolts won't fit through the slots either.

 

Thanks, Fishbulb28. So you don't think that an adapter crossbar is the best way to go?

 

Take a measurement or punch holes in a card to match the existing ceiling box to your favorite store. Use it to match up with something like this that will adapt:

or

 

Thanks, Bob22. That was my first instinct and will be the first thing I try this afternoon. Thanks again.

 

Drill the slots larger.

 

Thanks, everyone for your advice. I found a crossbar and it worked perfectly. After a couple of tries I discovered that the existing bolts were #10-24 and acquired washers and nuts. The light is now installed successfully in the old box!