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I have the LX mounts for my ACDs. They are fantastic. They come with a 75mm to 100mm adaptor plate in the box, so as long as you have a VESA mount on the Dell you are ready to go.
 
It should have the mounts on the back of the LCD when the stand is removed. Take the stand off, and make sure there are four taps. They should be pretty obvious, oriented to the center of the monitor.

If there are taps of either size, the materials that come with the Ergotron will sort you out.
 
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