Should have added that if you extinguisher is not Pyrene brand, it may not have Heavy Vehicle written on it. Other guys may know more about this subject, so I will invite further replies for you on this topic. While looking through my photo archives I came across a number of vehicles that had extinguisher which was not painted at all. Perhaps they were not universally painted in military colors after all.
This is an interesting site regarding General brand extinguishers.
http://www.vintagefe.com/general.html I bring that to your attention to show there is a fairly broad range of models & manufacturers which made these little gems. Some of these portables are just beautiful pieces. In hindsight, if I had no other hobby interests I would be very happy to collect & restore portable extinguishers.
http://www.vintagefe.com/pyrene.html Interesting Site!
I want one of these pistol style extinguishers from the 1950s. Man they had some cool stuff then!!