QUOTE:
The back plate retainer provides the threads. Look at the link from Newegg. Hover over the retainers for detail. They show both sides of the bracket retainer. The nuts embedded in the black plastic retainer is the bottom of the retainer. The other side that shows the threads that are raised goes through the 4 holes in the bottom of the motherboard. The four screws from the heat sink fan screw in through the top of the Motherboard sandwiching the motherboard between the retainer and fan heat sink for extra support. I take it you didn't build this computer or the bracket retainer was already in the motherboard when you got it. From your picture I believe the threaded barrels are still on your heat sink and came loose from the backing plate.
Thanks for your input. I built it so long ago I hardly remember anything. What I do remember is just pushing the screws down and they locked into mb. I never gave it a thought after that. I will look at the link. Thanks again for your informative reply.