Re: Old Fire Extinguisher
I can't say that I can offer too much on this one as I have never seen such a thing, but I'll put in my two cents worth...
Judging by the info you have given, it could be some sort of halon extinguisher. Halon systems are commonly used in locations where regular fire sprinklers would cause more damage than they would prevent by causing water damage (computer rooms, important document storage, etc). The halon system fills the room with halon, a gas that quickly displaces the oxygen, extinguishing the fire. If this is in fact some sort of halon extinguisher, I really doubt that it would be really harmful unless it is in a location that would make it hard to get away from it quickly if it were to go off.
If you are uncomfortable with removing it, I would say that the best thing to do would be to call the fire department and ask them about it. They may be willing to send someone out to take a look at it and advise you from there.