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
My guess is these became common with the telegraph. Since messages were expensive and the sender paid by the letter, abbreviations where commonly used to keep messages short.
Oh, another interesting take. Tysm for that reply :D