Hi, English isn’t my mother tongue so I was asking myself that question since I first encounted a w/… Back then I was like: “What tf does ‘w slash’ stand for?” And when I found out I was like “How, why, and is it any intuitive?” But I never dared to ask that until now
m. is not a German abbreviation for mit, afaik. I never once read that. Where did you get that from?
As an Austrian, I have often seen it on food packaging with limited space. Something like “Rotkraut m. Apfel”.