It is confusing with such similar symbols within the marks. As you noted, the mark on your set contains a caduceus, and for it to be right side up, the initials would be SI, not IS. The French are rigorous about the use of the symbol, so it is unlikely that this is Sussman, who is registered with a Tongs & Star for his symbol.
I checked the French mark database for all registered caduceus (caducÃ©e), but there is no IS or SI with this attribute. As it is, I believe the database only covers Paris or Ile de France in scope. Likely it is a maker from another part of France.