That stock--gray chipboard--is from a series of fakes made after ESCO gave up the ghost. While I cannot tell from the front because of the scanner moire effect, they are re-screens of genuine cards (halftone prints made from an existing card; put through the halftone screen process again), which you can tell with a side-by-side comparison to a real one.
There are two basic fakes: gray chipboard and bright white stiff board. Both have re-screened fronts. There are also a number of obvious reprints that show the cards with white borders on thin cardboard that no one will mistake for the real thing.
The only genuine cards with gray backs are the Canadian issues from the early 1950s and they are not re-screens. The Williams from that issue has a small number on the bottom right in a white diamond.
Please excuse the pinheads at SGC who misidentified this one as a standard card.