The cylinder must remain unharmed

What sound does it make if you hit it with a stick?

I can't read rusty foreign text very well, but if I had to guess, it probably loosely translates to something like..

'DON'T FUCK WITH ME, BOOM BOOM'

Boba fett vibes

It's imperative that the cylinder remains unharmed.

Green is usually for Argon or other inert gases.

That's a gas cylinder. Does it have a window shape on it like this ⊞? U find bottle still in use around Europe with that on it. The ⊞ used to be a swassticker but welded over and the cylinders still in use today.

The green colour makes me think it's a halon fire extinguisher.

Fire extinguisher the vibes tell me

Maybe an overground valve of some old stadtgas system