to expand on byoungblood's (correct) answer, it's a holder for hex drive bits, but it's wider in one dimension to hold a a spring clip to keep the bit in place. From the patent description:
"A U-shaped spring clamp plate is received in the second bore opening, and the U-shaped spring clamp plate is in an arrangement adapted for clamping and grasping the head of a tool or screw driver so that the head of a tool or screw driver cannot easily loosen off or rotate in the hollow sleeve during practical use.'