Also known as the "Sharktooth" cassette (for Mavic 571 freehubs, as mentioned.)

And yes that freehub has Mavic's own semi-circular splines so that cassette fits nothing but those.

(And even though it's not relevant to this issue, I'll just mention for information that Mavic 571 freehubs also came in variants with Shimano Hyperglide and also Campagnolo spline patterns.)