I thought BMW fitted brackets to the driver seat so as to keep the extinguisher positioned correctly .
I have only ever seen one but that is how I remember it was fitted .

X5 built in the US, but equipped differently for Yurrup...we don't get the fire extinuisher, first aid kit, warning triangle etc...
I am trying to figure out how to mount a plain old marine extinguisher under the front of the passenger seat..,....no snide comments please capish (?).....

You are correct, the holes are already there. You just need to tap them with threads. You can either use a thread tap or just the screws that come with the bracket and they will work just fine.

I keep my extinguisher in the glove box .

Can you aim me in the general direction of those holes? Are they on the seat bracket itself?
Having my non-pocket sized extinguisher fly out into my pedals somewhat defeats the 'safety' purpose.

Yes on the seat bracket. Did you look at the BMW installation instructions? They are very easy to identify from those instructions.