Thanks for the story of Jessie Roberta Cowan/Taketsuru. Obviously, I can’t help but have an interest given our backgrounds (albeit the MASSIVE disparity in our accomplishments!). It’s lovely to know that Kathleen Drew-Baker had a peer back then quietly making her own impact on Japan.

I bought my first tin of this kind of mustard in years a few weeks ago. It’s Colman’s Mustard powder but it is indistinguishable from the Japanese kind. Main reason was as an accompaniment to Hiyashi Chūka and for making cucumber tsukemono.

Assuming you use about a teaspoon of Karachi mustard at a time, each pot should give you around 20 servings. Another good way to go through it is as Karashi Miso Ae 辛子みそ和え. As well as cucumbers, goes well with eggplant, okra and asparagus.
There’s a recipe for Karashi Miso Ae here - https://www.nhk.or.jp/dwc/recipes/detail/264.html Just substitute a teaspoon of powder for the 2 teaspoons of mustard paste and then, once mixed, take a taste and then add another half or full teaspoon of mustard powder (or not) depending on how you like it.

EDIT TO ADD - responding inspired me to make up a hurried karashi miso mayo for a sandwich. 1 teaspoon of red miso, 1/2 teaspoon of mayonnaise and 1/2 a teaspoon of mustard powder, just combined and spread into the sandwich instead of butter. Lovely!