My grandfather was a machinist in a railway car repair shop. He made custom tools for his particular work. Some of his were steel and some brass. The pictured tool gives me the impression as being such a custom tool for a specific job. The jaws look like they will be parallel when they are about a quarter of an inch apart and the handles would be just open enough to easily grip when the jaws are parallel. So I'm thinking this was for holding some specific item about 1/4" thick. Might never be able to find out what it was.