Proof mark Id please

I just finished discussing Lugers with a fellow enthusiast in the USA, and he asked me if I knew what the following proof mark meant(he had seen the same one on a Luger at a gun show)....on page 213 of 3rd Reich Lugers there is a 1929 DWM ....on the RH side of the gun there is a proof? mark just above and behind the trigger on the frame...it appears to be a rectangle with a "m" above it.....I am most interested in knowing what it means....Thanks...