Beretta USA told me that the rear sights for the 92A1 are not just pushed in but they are staked. They do not have any diagrams to show, though. She said the stake at the bottom of the rear sight on the left.
I cannot see any pin hole visible on the outside that is connected to the rear sight.
Where on earth is the stake driven in from?
Can someone throw some light on this, please?

It is not what you are thinking. To stake something is to peen over a portion of the metal to prevent movement.



 

The rear sight on my 96A1 is not staked, and I've never seen on that was either.

Look at the top of your slide.  About halfway between the breachface and the rear sight you'll see the hole where the extractor pin is located.  There should be two small indentations opposite of each other on the edge of that pin hole.  Those are the staking marks for the extractor pin, and that's what staking typically looks like.  

If the rear sight on your 92A1 is staked there would be a small amount of metal around the sight dovetail displaced over/into the sight itself, similar to what you see with the extractor hole & pin.

I've got a 92A1 and the rear sight isn't staked at all.  Mine was removed easily using a sight pusher.

I've not see a Beretta 92/M9 with a factory staked REAR sight.  They are simply installed in a bare slide with a press, and then parked and painted.  Yes the rear sight is tough to remove but I've not seen one staked as well.  As long as you remove the rear-sight from the RIGHT to the LEFT (as looking down the sights like you were shooting the weapon), you just need a bigger hammer or a sight-pusher.  Good luck.