I'd call that a bent tail lathe dog. It is easy to devise a dog that works just as well by making up two bars with screws to clamp them together. Extend one bar enough so you can put in a bolt, perpendicular to the clamp bar and long enough to reach the slot in your driving plate.
Buy a cheap machinist's parallel clamp and tap it for a bolt to use as a drive dog.
For smaller work, buy a ground rod clamp which will already have the hole for the drive dog.
Three dogs I quickly fabricated. Not the most elegant, but they all do the job. With a little effort you can do the same or better. And faster then it takes to find and purchase.