It's a Swiss can (not a flask) used to hold denatured alcohol for field stoves, not the kind you drink. I think it is post WW2 but that's just a guess.
In the US it is called denatured alcohol. In Europe, it may be called spirits, brennspirits (Germany), alcool a bruler (France), or alcool etilico denaturato (Italy).
The same name may apply to the Swiss, as 'diggummup' mentioned above but, then again, the Swiss seem to talk half a dozen languages--German being one.
Don...