I do not have a Billings catalog, but perhaps their numbering system is different than others. In my 1950 Williams catalog, the No. 500 is a single end set screw wrench, 3/16 inch, which is consistent with other catalogs that I have (Bonney, etc.). Your wrenches are both engineers' wrenches. In the Williams, the 3/8, 1/2 would be the 723A.