It was pure aftermarket. As mentioned, it was an outfit called Wilcap, out of Torrance CA. Here's the article (downloadable PDF...I've configured it so you can zoom in for E-Z reading).
IIRC, MA mopar investigated converting the /6 to diesel.
Yes, and about the only thing the turbo diesel Slant-6 had in common with the gasoline-fired one was bore, stroke, and six cylinders at a 30-degree angle. Chrysler got very far along in the process (somewhere past prototype and into preproduction testing -- see here) when the plug was pulled. It was a great deal better engineering job than GM's halfassed work on the Olds 350, and that slack garbage by GM is what torpedoed the Chrysler program; see here (search page for diesel).
