Does anybody know what the name of this feature is?

This is the back side of a hyperglide hub where the hub body engages the wheel hub. I'm trying to upgrade to 10 speed. Original on the left, new part on the right. There's 10 teeth on the old one and 15 on the new one.

New one obviously won't fit due to the difference in the number of teeth, but adding 10 tooth to my searches doesn't bring up anything helpful