Now you have.
To those of you who were hit by that choking sensation when some metal thing fell next to you, this is what holds the gas chemicals.
This particular one was manufactured in 2005.
Apparently, when the canister is fired, the top comes off and the gas is released from the hole in the middle.
The corrosion at the edge of the cylinder head should be sufficient to demonstrate how potent this stuff is.
Thanks to Sekaran for displaying his prized memento :-)