The vinyl was indeed made (and patented) by JVC. The trade name was Supervinyl and it was made using virgin material in a proprietary formulation. MFSL used it exclusively until JVC stopped offering it. Others that used it were Japanese home-market labels (often referred to as YSL), Barking Pumpkin (Zappa) and Nautilus. Some have ascribed its withdrawal from the market to declining volume, others to the truly expensive, nasty process necessary to make it. Either way, JVC has refused to sell the patent or license it to anyone else. A pity; it wears like iron and is the quietest formula I've ever heard.
Early MOFI vinyl formulation
From my memory, MOFI's early vinyl was sourced from JVC Japan? The most obvious trait is, you can hold one of these lps up to a light source and actually see light through it. How is this possible from a black vinyl lp? Why hasn't any other lp manufacturer tried to replicate this? One thing for sure, this vinyl was/is quiet!