Pen guns have (officially) been around since 1925, and were likely in circulation well before then. And by “pen gun,” yes, I do mean a gun-shaped to look as if it was a pen.
While most in-line guns of this sort are illegal if they are not made on a Form 1 (and a tax paid) and transfer on a Form 4 (with another tax stamp) under the AOW section of NFA-regulated devices, there was a breed of transforming pen guns that morphed into a traditional Title I pistol and needed no such stamps.
Behold, the Braverman:
More on this bad boy, which I recently got to play with, in my column at Guns.com.