BHM stands for Bridgeport Hardware Manufacturing, which was set up in 1895 as a maker of various hand tools. This 9 inch auto wrench was made into the depression times at least, which seems late to me, i don't find this style of adjustable wrench very useful on hex nuts. They do look nice though!
The company made wrenches, pliers and screwdrivers also and in the 1930s became a contract manufacturer for Snap-On. According to the always thorough Alloy Artifacts, the company was bought by Crescent Niagara in the 1960s, who continued to offer BMH tools for a number of years.