Here is the 200-inch disc in the optical shot at Caltech on January 3, 1938*. That is a 104-inch grinding tool sitting on the huge Pyrex disc.
In this view from the side you can see through the edge of the glass and make out the honeycombed structure that forms the underside of the disc.
Thanks go out to Nancy who has been helping with our scanning of this and other vintage Palomar images!
*Curiously enough the next photo in this archive shows an optician working on a smaller piece of optics. On the wall behind him is a 1939 calendar, so it may be that the image above has an incorrect date.