Sigurd,
Older M2/3 cases in Europe do have mostly a thick ('German') screw, and do not fit the M6. However, in the US, on ebay, you can find many M2/3 cases with 1/4 screw thread ('English'). They fit very well and when you find one in a good shape (about $50) you have a beautiful thing. Their drawback is that the snout is fixed and can only be removed by using violence (saw). Using only the part in which the camera is fixed by the screw keeps your body clean (no wear on the strap carrying eyelets: the strap is connected to the case). The cases which have a provision for carrying the camera with lenses with 'goggles' (35mm and 50mm DR) are the best, they offer extra protection at the right side of the camera where its most fragile part is located: the viewfinder.
Best regards,
Erik.