One of the makers of tear gas "pen guns" was Federal Laboratories, of Pittsburgh, PA, and I think that is one of their products. Now part of Defense Technologies, they are still a major producer of non-lethal weapons for both the civilian and police market.
Most of the tear gas pens produced since the 1930's had the gas cartridges soldered or otherwise fixed into a stub "barrel" whch screwed into the body of the gun. That way, a normal cartridge could not be fired, so the pen gun was not a firearm. If the pen gun were altered to fire a normal round, it would blow up and injure the shooter.
The cartridge case head shown appears to be fixed into a "barrel" in that manner, so it is probably not a firearm.
Jim