uniqueid198x

In the Speedwriting shorthand system, developed in 1924 for use with typewriter, / Is used to denote omitted sylables, so ‘with’ becomes w/ and ‘without’ becomes w/o. Here is a pretty deep guide on the precepts of Speedwriting:

https://www.reddit.com/r/Classic_Speedwriting/wiki/list108/

Yeah. I have no evidence that this system invented those shortcuts, they may predate it by quite a bit