That would be a Buckingham thread restorer, probably a 6203. It's a universal thread restorer, the 6203 has a capacity from 1/4" to 1". Short version, you open it up wide enough to slide it past the damaged thread, tighten it down so the little wheel rides in the thread, and turn it off the part. The serrated jaws push the metal back into shape, restoring the thread. I have a 6103, basically the same tool, but with one longer handle rather than the two short ones, the 6203 is meant for use in a confined space. I've had it around for a long time and only recently had the opportunity to use it and it worked fine. Good find.