without a pic, it's hard to identify, but look in a library for a Limoges porcelain book. The style of the mark, initials over a line, over France, is somewhat typical. There were at least 35 companies in the Limoges area, during the time period of the cup. I hope you find yours. If the people mentioned in the signature are your ancestors, what a treasure. Many porcelain makers sent their wares to decorating studios. (earlier version of "outsourcing"?) And in that way, yes, it's possible the insignia could be under the glaze. KB