YES. Marshall did inquire about a quad master tape for "So What" and it was not available, found, located, or even confirmed as ever existing. The only evidence we will ever have is the Koss Demo Disc.
I love how it's always an ambiguous answer and never anything definitive like: "Damaged", "Unusable", "Lost", "Unaccounted For", "Incomplete", etc.
Always left for interpretation probably due to either liability issues (like getting sued if the artist learns the company lost or damaged a tape), or they just love dangling the carrot giving us hope.
Problem is, the clock is ticking, for both the tapes and the market for these recordings. Strike now while the iron is still luke warm.