How does this heatsink come off?

I have an old P4 machine that I'm trying to breathe new life into with a cheap motherboard/gpu upgrade. I've taken it all apart, but can't figure out how to get the old heatsink off so I can move it and the CPU over to the new motherboard. Pushing here, pulling there, even a gentle twist - nothing. Google's been nearly useless - every P4 heatsink guide I find has simple-looking clasps, nothing pictured is like this thing. Has anyone seen this before?



More views: http://img21.imageshack.us/gal.php?g=p4heatsink004.jpg