The word "casse-tête" literally means head crusher but is used even in law texts to refer to knuckle buster.In france, any lockable blade is legal to possess but illegal to cary as it is considered as a sixth category weapon, category wich includes any fixed blade, any lockable folder, knuckle busters, pepper-spay,sword canes, crossbows and many other. Basically,everything but slipjoints and friction folders is illegal to carry.
Would your design be considered as illegal? Worn as a ring, I think certainly because the function would be obvious,worn as a key-hanger maybe not but it quite defeats the function... Anyway, the final appreciation of any object fitting in the sixth category or not is left to the policeman arresting you, wich is very bad as french policemans are prone to overzealousness and abuse of power.

I personnally got robbed by french police, they took a very nice damascus slipjoint folder from me saying the blade was too long but didn't make any form of legal statement and send me away argueing some legal problems as I was discussing...
