I saw it on the book Mechanical Typewriter that I bought a few years back. It is supposed to even up the right hand margin. According to the author,Thomas A. Russo, it was manufactured by Jusifier Sales Company in Los Angeles and could be mounted on most typewriters.
Very ingenious device. My hat off to its inventor. I've always wondered how typewritten tests can be evened up on both margins, except calculating the exact number of strokes in each line. The Justifier seems to be the solution. But I've never seen it on eBay, so it must be rare.