if you take the book on page 417 you will see an "S" but in mirror.
The add speaks of "engraved" initials.

I see no use what so ever to make chips with initials in mirror so my best guess is that the catalog just shows the artwork, not the actual chip.

My 2 cents and gladly learning more if my deduction was incorrect.

All the best
Jo from Belgium.

Same time (1880), different type of chip.