The Krummlauf. Or, more correctly, the Vorsatz J (or I) and Vorstaz Pz. Originally a way (Pz = Panzer, 90 degree bend) of letting tank crews engage infantry without sticking their heads above the open hatch. Also developed (I or J, 30 degree bend) for infantry in trench or urban warfare.
Not sure it ever saw service. Never replicated by any other army. Which tells you something about its utility, 72 years later.
Back on topic. Tap-loader, anyone? Surely it's not just me?