Thanks for sharing this information. I recently came into 3 piece tea or coffee service with this mark and am wondering if you have the active working period for this mark?
So far I have found a few pieces with general attributions to the 20th century, and 1-2 saying late 19th century.
monte100, the 'plant' you mention has been described as a tuft of reeds on a dealer's site:
Société d'Orfèvrerie du Marais : Ste OM avec touffe de roseaux
Société d'Orfèvrerie du Marais: Ste OM with tuft of reeds