Power Cube 4 wont fit in 7900 series IP Phones

Here's an interesting item I've just discovered and worth remembering:

Is CP-PWR-CUBE-4= compatible with the 7900 series phones?

A: No this power cube is for the new 8900/9900 phones, compatible with Communications Manager. The power tip is different size, won't fit in other phones.

So now we know!