Never seen it before, but perhaps others have. With your permission, I could forward the question to philaseiten.de - there may be some who know the answer. If I may venture a guess (it's a long shot), the abbreviation could stand for "Feuer-Societät Dresden", an old fire insurance company. But don't rely on it. It's a pity the postmark isn't clear enough to make out the place. That would be another clue.