Removing the metal backplate is simple. This is just stuck on with some flexible glue. So prise something under and off it pops.

Inside you can see a number of large cut outs in the rear of the plastic casing.

Note the small pinhole in the metal backplate that aligns with the rubber sealed hole in the plastic casing - more on that later.

There is also a very small ground out area inside the metal backplate that appears to align with the rotating mass of the vibrator.