Manolo,
As you noted and I completely agree with you, they can be from anything. The only requirement is that whatever was shortened, should have somehow made its way to the Maghreb.
Here is a picture of a Bertheir bayonet from Richard Abbenbroek's web site, where he has uploaded pictures of various bayonets from his collection. Look at the peculiar discontinuation in the fuller towards the base of the blade - it seems that the same feature is present on Charle's genoui, so his must be made from such a bayonet for sure.
Of course, other bayonets, and not only blades, were also used. I have a shula dagger with a scabbard mouth made from a bayonet scabbard.
Regards,
Teodor