This is one of my mother-in-law's hand-written recipe with one very obvious omission, even in the ingredient list - the cabbage itself! I guess she thought any decent cook would know how & when to add it. Still, it caught me by surprise & I had to think about it for a minute - I wasn’t sure if cole slaw had been made very differently in previous days.
That being said, I’m not sure when to add the cabbage. I've added to possibilities.
I also suspect the author may have been referring to dry mustard; I’d experiment with both.
My additions to the recipe are in parenthesis.