A frame member fastening device and method of manufacture provides a single configuration fastening device which can be used singly in connection with a frame member, or which can be combined with another identical fastening device to form a fastening device assembly for use with frame members of different internal widths, by interlocking two identical fastening devices in an opposed, base-to-base assembly configuration. The fastening device is manufactured in a progressive metal forming process with successive stages in which the features of the device are formed.