It should have a number on the other side of the head. It is probably a 20B, 20, or 20A with an opening of 4 1/2, 4 5/8, or 5 inches. In the 1960 catalog, there were 5 wrenches larger than the No. 20A, with the 22B having a 7 5/8 inch opening and 52 inches long.