I recently cleaned out a huge bike parts collection, including tools, I found several of these in the guys tool box, and mixed in with some hub parts.
Its marked only in 1/8" increments, its flat on two sides, and has a 3/8" stud and nut on one end. The larger threads are 5/8" in diameter and is marked from 0 to 4 1/2 inches. I also found another one of these in another cleanout a few years ago, I tossed it in my tool box but have never found the use for it.
I pretty much am assuming its originally a bike tool since all of these I've come across came from bicycle hoarder or home bike shop cleanouts.
I know I've seen these before somewhere but can't recall what it was used for.