I recently turned up a 7.65mm Luger of Swiss WW2 vintage, from when they were semi-experimentally making aluminum cases. This one is headstamped “M 3 T 44” and has a blue aluminum case, with “24” in what looks like mechanically-printed black in on the case side. Flat brass primer which is unfortunately snapped, plus a standard CNCS FMJ bullet.
Somewhere, back in a dim recess of what passes for the Wombat memory, I believe I read that the blue case indicates a proof load. Can anyone verify?