What is the subjective wellbeing instrument that is used by Science of Happiness for check-ins? I am doing some work with students on happiness and thought it would be useful for us to have a look at their subjective well-being.

The EdX Science of Happiness course made use of weekly checkins like this one:
https://ucbpsych.qualtrics.com/jfe/form/SV_aVt9kWutzGlImwJ

The test is 6 questions, each one you are shown a drawing of a face with an emotion (3 positive, 3 negative) and you are asked to rate "To what extent does the character above match how you've been feeling lately?"

I'd like to learn more about this instrument. Like what research has been done to determine the validity of the measure, the origins of it, considerations when applying it.

I can't find any information on it, there doesn't seem to be a reference on the course page as to where it came from.

Does anyone have any ideas?
The background of the measure can be found here.
The article implies that it is an ad-hoc measure of emotions created for this study:

Every week, we checked in with our students to see how they were feeling. We showed them a sequence of six cartoon faces created by Pixar artist Matt Jones to convey fear, enthusiasm, anger, affection, sadness, and amusement (below). Under each, we asked them to rate, on a scale of 1 to 10, how much each face matched how they’d been feeling lately.  Then we transformed their collective weekly ratings into a single score.



The result? For students who responded at least 8 out of 10 times—suggesting that they were fully participating in the course—positive feelings went up, and up, and up. They felt progressively less sadness, anger, and fear, while at the same time experiencing more and more amusement, enthusiasm, and affection.
We also invited students to fill out a brief battery of research-validated questionnaires that are regularly used to assess feelings like happiness, stress, flourishing, or satisfaction with life.

The last sentence suggests the ad-hoc nature of the measure (as compared to validated scales). Also, I couldn't track down any published research with this measure. The icons themselves stem from a collaboration of Dacher Keltner with Facebook, as described here, for example.