Thanks for the further input Jim - the marking is a puzzle is it not!
My feeling about the figures you show is that whilst the marks are not perfectly represented, 3 of 4 are clearly very similar to the original they are supposed to be copied from, but our focus mark is really a stretch. Given that the other examples we have of this mark are all relatively similar, would we expect that the artist messed the original up so badly but that everyone else could copy his work so well? So I'm with you on this, not even picasso would produce representation of the fly mark like this. The long vertical marks (staff) are also on all the examples I've seen so far, which does not bear any similarity to any of the European stamps - so I'm inclined to think this mark represents something else.
Poking around in the forum threads I came accross this one:
http://www.vikingsword.com/vb/showthread.php?t=11950
In this thread RDGAC posted four kaskara from his museum collection, one of which both bears the 'comet', similar fuller and size of blade and has an identical cross-guard to my rusty example. Unfortunately this one was precisely the one that did not come with any guarantee of date collected
But if it was as suggested in the early to mid 1880s, then we have an example of the mark on a kaskara with a date comensurate with Briggs' takouba example.
I wonder if RDGAC has dug up any more information since this thread, it would be extremely useful?
Looking at the hilts on both my swords - the croc grip kaskara has an almost identical guard to RDG examples 1 & 2, but none of these look like they are formed from four parts (no X, although his No3 example does have one), so I wonder if that can be used as a reliable benchmark for dating? As Iain noted the croc kaskara guard is of high quality and does appear to me to be old. Incidentaly one of the RDG swords also has the iron peg clearly visible that was not used on my rusty sword's hilt, hence why I think the grip was perhaps replaced at some time.
Chris