Safety advice – WW2 oxygen bottle

Hi all

I am sure this has been asked before but I am not able to find anything.

I have a WW2 period British oxygen bottle that does not have a lever or knob fitted to turn the supply on or off.

How can I identify if the bottle is empty, and is it dangerous if it is full?

The mechanism has what looks like a screw inserted down the top – there are outlets on either side but they are blocked off. The screw could be where a Bakelite knob has broken off.

Would I be safe to unscrew the screw?

Will post a photo later if you cannot understand what I am trying to say.

Cheers
TEC