These were available for general sale when I was considerably younger. They were even advertised in the back pages of comic books.
Since they were for sale during my lifetime, I must note that I am deeply offended by being referred to, even at second hand, as an "antique."
I believe that these pen guns were supposed to be used only with tear-gas cartridges. However, I know that people did use them to fire bullets.
I probably would not do that, were I to come across one of these devices.
Some of these pen guns have trigger buttons, and in some cases the trigger button is actuated by the thing's pocket clip. Some fire only by pulling back, and then releasing, the cocking knob. The little "side track" off to the right of the cocking knob is the safety notch, such as it is.