One minor correction:
I mentioned these architectural slabs to Ray Annenberg on Sunday September 27 at the Cambridge Glass Fair, and he recalled making some himself, so this example could have been made by him. He kindly explained to me that the mould was just the four sides placed on the marver, which formed the base. I had erroneously assumed that the mould was box-shaped, i.e. with a base. Also the mould and the marver weren't lightly textured but flat.
Grateful thanks to Ray. :hiclp: :hiclp: :hiclp: