Okay so while my data connection was down, Perhaps this is some evidence, Internally
I looked over the
FCC 521 Pics (Select Internal Photos)
Download in 520 Service Manual. go to Page 11
Visual Pic of Internal showing difference between 520 and 521.
Where the camera is located at on the 521. To the Right of the Camera Lens itself some type of sensor, On the 520 does not exist. So that extra notch right below camera on shell is for something after all.
The extra length on 521 probably comes from this. One of Chips along the bottom is arranged differently on the 521 then it is on the 520.
On 521 is running left to right. On 520 you can see that is running up and down.
You can tell from the pics that 521 board is different than the 520. Notice the notch on the 521 right above that chip running left to right, not on the 520.
Now the Mystery Is Solved I believe
your thoughts.
I would post edited images from the pdf but they are copyrighted.