I think that may be a good resolution, that this mark (enigma) might reflect as award or perhaps as you suggest, a level of achievement or status in a particular Tariqa in Sufi. That would explain the relative rarity (as far known) of this symbol on these sword blades.
In support of this seems to be the fact that in Briggs, the examples he notes having this device seem to only have been those of notable Tuareg chiefs or extremely prominent figures. As these are described naturally they are only seen through the filter of known European blade markings and native interpretations of them. Frankly that was in degree the level of consideration I was operating on as well, along with possible talismanic or folk type associations in symbolism.
The idea of a high ranking Sufi associated emblem is further supported by your mention years ago of the Grand Master of the Grand Lodge in Egypt receiving a kaskara with it on the blade in Khartoum in 1961.
That appears to be the only mention of this enigma mark beyond the three in Briggs. Here again I note your observation that these are apparently engraved skillfully by an artisan likely in precious metals rather than a native armorer dealing with more pedestrian level swords mounted with trade blades.
I think it will take a great deal of theological research in these elements of Sunni Islam and Sufi Orders to gain knowledge of the meaning of the enigma mark, and that certainly exceeds the general knowledge core in the average arms community. Possibly some of the scholars who have written on these religious aspects of arms in Sudan might have such awareness.
For our purposes at this level, as you note we need further exemplars of this mark on blades of other takouba or kaskara to test the exposure levels of the use of it, as well as hopefully provenance or additional context that might offer more insight.
Apparently the examples shown by Briggs of the Adembar and Raidera swords are in the Musee d' Ethnographie of Neuchatel (XXa, b)
It is unclear where XXc is. I have the Gabus reference pending, and have located Morel, Lhote for referencing so will see where that goes when all together.
Then maybe seeing if the Neuchatel museum has the examples, maybe photos of the markings?
It would seem that the 'hatched' marks (enigma) Briggs refers to as 'bees' have a connection to the stamped versions of the 'cow head', style example (as seen on the blade of the Darfur kaskara I posted) if the second one in XXb with two loops to the side is considered as transitionary to the stylized 'hatched' example.
The game is afoot
