The bottle to which some of you are referring is indeed fixed to the forward bulkhead of the galley...Under normal circumstances it would be operated by the AEO...After the bay has been checked and fire confirmed by one of the back enders via a shufty scope (small black item that you shove into a small access hole in the bay) the AEO will ensure the extinguishers hose is fitted correctly into the appropriate bay's 'hole' on the bulkhead next to it...The holes are labelled H, A or E for the three bays...Then he'll give the trigger a ten second press and ask for a report from the bay...if it's still on fire another ten second press followed by a further report...if it's still on fire then another press but this time unless they've changed it recently the AEO will go till empty as the thing only holds enough for about 3 times 10 seconds presses anyway...
Hope this helps.