Two diagrams of the Commander's (CDR) and Lunar Module Pilots' (LMP) PLSS Tool Harnesses.  Compare with this.
The harnesses were designed to carry the equipment that the other crewman would use on the moon, since it was impossible for them to reach behind them while wearing the suits.  Hence, as the CDR was usually the one designated to do the hammering, this tool was carried on the LMP's harness, and vice versa.