So I picked up this cassette recently. I know nothing about it. Do you know what it is? Please let me know. Thank you.

Mavic 571. Freehub is still attached. Note the thread-on first cog. IIRC, Mavic's attempt to develop an alternative to Hyperglide. Good thing this never took off.

Is that the same threading as Uni Glide?

Suntour's last ditch effort to stay relevant included a similar design. I only worked on one and it was a battle to get it off; I recall breaking a tooth on the threaded cog before it finally cut loose.

Don't know but I doubt it. The last thing that the larger bike and component brands would want is to introduce cross-compatibility into their systems.

I do know that the Mavic freehub bodies had circular-shaped splines instead of the sharp-edged splines of HG, so I doubt there was dimensional compatibility there as well.

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.)

Here:

Urban legends, and things I was right to believe in... ~ tears for gears