The unusual trait of these securities (to modern eyes, at least) is that the company sold them at less than face value. However, there was a catch: the company could come back for the rest of the money later if they needed to (See: Assessable Security). There wasn't a lot of rules on when the company could do that, either, although they could not ask more than the full value of it...at least the amount wasn't a surprise when they asked.
The category includes assessable capital stock. With this kind of security, in the event the company closes, the investor can be liable for more than what they bought it for (so the company could call in the rest of the value). Today all stocks on major exchanges are non-assessable, and it has been this way since World War II.
