I've had the good fortune of acquiring some UltraStable pigment tissues recently, and moreover, the opportunity to communicate with the inventor/creator of the system, Charles Berger. http://charlesberger.com/
In a nutshell, this was the last commercially made color carbon process, ceasing production in 1999. It was a CMYK color scheme and the tissues came pre-sensitized.
I recall reading somewhere that the pigments were adopted or inspired by the automotive paint industry. I hope that this can be elaborated upon. They are by all accounts incredibly permanent.
As you can see from the swatches, the pigment sheets are very pure and deep in color, but they reveal more typical "process colors" as they thin.
To test the tissues, I cut two slivers and exposed one to UV light and the other went straight to my faucet for a solubility test. Sure enough, hot tap water dissolved the pigment away and confirmed that the sheets have not fogged/hardened. The exposed sliver did what you would expect, and did not dissolve.
As for emulsion batches; the cyan sheets are from 410, magenta from M523 and yellow from Y334. Unfortunately I'm missing the key sheets.
I'm excited to learn more about this process, and will be posting links and articles as I find them. Charles has offered to post the original price list and working instructions for the process on his website, in addition to providing a reading list. He has so far recommended Le Vocabulaire Technique de la Photographie (Marval, Paris, 2008), a French book with a comprehensive write-up.
A couple of points that I'm most curious about are as follows: Was there a UltraStable brand receiving paper, and if not, what was the recommended receiver, and/or the means for its preparation? What is the sensitizer? (I don't know if this can be disclosed, but there is no stain that I can observe, and the keeping qualities greatly exceed a typical dichromate sensitizer) Also, sensitometric data and recommendations for masking would be interesting to know.
I look forward to learning more about this amazing printing technique!