There are a bunch of different brands, sold over the last century. The ones made by Pengun didn't shoot a fixed projectile. They were all gas guns or flare guns. NATO also sold flare guns and signal cartridges (like an M80 that flew). Those are orange with a floppy holder for the 9 or 10 cartridges.
ADC (American Derringer Company) used to be in Waco, TX. I think they moved up to the Canadian border, St Ste Marie, maybe. I've got a couple of those, one in 22lr and the other in .25 ACP. To fire them you've got to "bend" them to cock. Then a little trigger that you push, not pull drops down. Its actually shaped like a hand gun in its fire position. If that's what some here are talking about, I'll take them to the next gun show! Yes, they've become rare these days. The 22 caliber gun can even be fired with the huge assortment of ammo, including 22 shot loads. The .25 is probably the better "man stopper".
Also in the past we saw a bunch of tear gas only pencils. Fully functional mechanical pencils with the eraser end a .32 cal tear gas loading. I used to have one of those, but we moved and I didn't see it. I had fear at one time it might be one of the dreaded "AOW" and in need of a tax stamp. Guess its gone so I need not worry about that in the future.
As I understand the distinction (you're a fool if you trust me), a pen shaped firearm that shoots a fixed projectile is against the law. What makes the Pengun legal is it fires a signal device, be it a flare or report. What makes the ADC offerings OK is the fact you need to reconfigure it to a gun shape to make it fire. What "might" make the old gas guns illegal is you don't have to reshape them and in theory at least, they could be modified to fire fixed handgun ammo.