Seen at a presentation about sustainability at LEGO House in September 2017. See this New Elementary article.
The bricks (made with a new plastic, but not the final solution yet) have “test” written on seven studs and 3, 4 or 8 written on one stud (the fifth, starting from the top). On the back there are thin and thick variants, one with multiple cross supports.
Disclaimer: I don’t own these, unfortunately, they just showed them to us and let us take pictures. 🙂