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That top firearm is equipped with a traditional stock and thus it is classified as a rifle.

The botton firearm is equipped with a "brace". Which in that particular example is far, far more like a traditional stock than it is an actual brace (see below). But nonetheless, that brace magcially turns the firearm into a pistol.

Early stabilizing braces looked something like the one in the link below. They permitted disabled persons (many of whom were veterans) to use firearms one handed / one armed. And pretty quickly, people discovered that many braces could be shouldered and used just like a more traditional stock. And just like that, genie was out of the bottle. Braces gradually became more and more stock-like over time. The brace in your example is a brace simply because it doesn't have all of the surface area against the shoulder that a traditional stock would. But it has plenty to get a good, steady shouldering of the fireamr.

https://thril.com/pistol-stabilizing-braces-history-braces/