Ian, i wasn't necessarily suggesting that this dagger was of masonic origin, however, with all due respect to your own personal experience with masonry, skull imagery has long been a part of mason ritual. Just one example would be the skull and cross bones on the ritual apron of the Knight of the Rose Croix. The apron of the Master Elect of Nine depicts a bloody severed head held by a hand and arm by the hair. Animal imagery abounds, from dragons and snakes to lambs and two-headed eagles. I also am not aware of any of this imagery being used, per se, on any of the masonic ritual daggers, but many magickal lodges took clues for their ritual structure from Free Masonry. These groups were not in any way satanic.
I must disagree with Jose. Satanic cults were in no way "in vogue" during the French Revolution. Satanic scares were. Catherine La Voisin's cult, where it all started, at the court of King Louis XIV was probably not all that large or as terrible as was made out. It has always been very easy for any one to cry "SATANISM!" at anything the slightest bit heretical. Just look at the history of the witch trials. Most of the so-called satanic cults were nothing of the sort. Even Aleister Crowley, the self appointed "Great Beast" of the 19th-20thC was much more a Christian at heart than he would ever dare let on. Of course, some people took his antics far too seriously in the 1960s and contemporary "satanism" was born.
Sorry for the digression. It all just to say that i still feel it is much more likely that this dagger was made for a member of a non-satanic magickal lodge. They actually tended to take there magickal work much more seriously and i believe would be more likely to have such a beautifully done piece of work commissioned. Of course, if the inscription reads "All hail satan" i will definitely concede i am wrong.