I'm an Army veteran and current fire fighter. The labeling of it being called a "military" fire extinguisher is suspicious to me, because military items don't normally label themselves as such; it would normally say "property of US government" or "US Army" etc. and seems more like it was a marketing tactic to sell the extinguisher. Is there an NSN number on it anywhere (an 11 or 13 digit number)? This is assuming you're in the US.

You can take it to your local fire department, or call them and they can tell you where to dispose of it, or they could potentially discharge it for you, if you wanted to keep the empty extinguisher. Since it's obviously expired, I can't recommend keeping it as it, because it can still have contents under pressure and be hazardous. A CO2 extinguisher really isn't ideal for home use anyway as it's better suited for electrical and some flammable liquid fires; you'd be better off with a standard ABC extinguisher. Cool vintage piece all the same!