The Apple VESA mounts for the older 20/23/30 monitors have a different part number then the one for the newer 24/27 displays. (M9649G/A vs MC434ZM/A). Not sure what changed.

The opening on the back for the stand is 4 inches wide, and the provided stand width is 3.8125 inches wide at the point it enters the monitor.

Found this YouTube video of someone installing the bracket on a 24 inch monitor if it helps.